Do you mind if I ask what makes you arrive at that proportional larger size than humans? I generally just assumed human-sized (or smaller, to make it proportional to dwarves) torsos when I was going at it.
I'm also still curious as to what sort of culture and role in the world you see nagas having.
OK, let me see... very roughly, half of a person's height is in their torso and head, the other half being in their (non-existant in a naga) legs. 5/4ths proprtionality means they would have a body like that of a 7 or 7.5-foot-tall human on average (with "large" nagas being more like 9-foot-tall people). Assuming we'd need about 3/4s of the mass of a naga on the ground pushing that torso along that doesn't help with locomotion, and an almost 4-foot long torso, we're talking about a length of about 16 feet, minimum (up to 20 for some of the larger nagas). To be faster, or survive having a bit of tail cut off, you'd probably want more of the body focused on locomotion. Also, coiling around someone pretty well probably works better with longer, thinner body types.
Very roughly, I'm guessing this is a total mass of about 110-120kg on average. That's something like [SIZE:115000], by the way, since size is measured in either mL or g, and the game seems to functionally assume organic creatures have a density of 1L/kg. That's a size double that of a dwarf, so you might want to drop the strength ratings on some of these nagas below what you would otherwise do, otherwise it might just be unfair when playing as a naga.
For the record, this was the code I had for lamias when I made them:
[BODY:LAMIA_BODY]
[BP:UB:upper body:upper bodies][UPPERBODY][CATEGORY:BODY_UPPER]
[DEFAULT_RELSIZE:1000]
[BP:LB:lower body:lower bodies][CON:UB][LOWERBODY][CATEGORY:BODY_LOWER]
[DEFAULT_RELSIZE:600]
[BP:UTAIL:tail base:STP][CON:LB][STANCE][CATEGORY:BODY_LOWER]
[DEFAULT_RELSIZE:1800]
[BP:HD:head:STP][CON:UB][HEAD][CATEGORY:HEAD]
[DEFAULT_RELSIZE:300]
[BP:RUA:right upper arm:STP][CON:UB][LIMB][RIGHT][CATEGORY:ARM_UPPER]
[DEFAULT_RELSIZE:200]
[BP:LUA:left upper arm:STP][CON:UB][LIMB][LEFT][CATEGORY:ARM_UPPER]
[DEFAULT_RELSIZE:200]
[BP:RLA:right lower arm:STP][CON:RUA][LIMB][RIGHT][CATEGORY:ARM_LOWER]
[DEFAULT_RELSIZE:200]
[BP:LLA:left lower arm:STP][CON:LUA][LIMB][LEFT][CATEGORY:ARM_LOWER]
[DEFAULT_RELSIZE:200]
[BP:RH:right hand:STP][CON:RLA][GRASP][RIGHT][CATEGORY:HAND]
[DEFAULT_RELSIZE:80]
[BP:LH:left hand:STP][CON:LLA][GRASP][LEFT][CATEGORY:HAND]
[DEFAULT_RELSIZE:80]
[BP:LTAIL:tail length:STP][CON:UTAIL][STANCE][LIMB][CATEGORY:TAIL]
[DEFAULT_RELSIZE:3600]
From the caste definition:
These tags establish the growth phases of the creature's life. The format is (BODY_SIZE|<year>|<day>|<average size>).
[BODY_SIZE:0:0:3500]
[BODY_SIZE:2:0:20000]
[BODY_SIZE:15:0:65000]
Basically, I made them fairly small (I'm actually a little surprised how small, relatively), so they would fit in more with the other castes they were living with.
Their serpentine portions took up basically 3/4ths of their body length - they should have been about 10 feet long, so they would have had a "human" portion 2.5 feet long, basically being 5/6ths the size of a normal human (which I guess comes out about right compared to dwarves and the sidhe).
Physically, lamia are strong and fast, but lack endurance or toughness. They are also social creatures, but lack the mechanical awareness of Dwarves. If Dwarves are Engineering, lamia are Marketing.
Lamia will have a SPEED value, because this scales with stat growth better than simply giving them high agility.
[SPEED:800]
[PHYS_ATT_RANGE:STRENGTH:700:1200:1400:1500:1600:1800:2500] ++
[PHYS_ATT_RANGE:AGILITY:450:950:1150:1250:1350:1550:2250] +
[PHYS_ATT_RANGE:ENDURANCE:150:600:800:900:1000:1100:1500] -
[PHYS_ATT_RANGE:DISEASE_RESISTANCE:150:600:800:900:1000:1100:1500] -
[PHYS_ATT_RANGE:RECUPERATION:150:600:800:900:1000:1100:1500] -
[MENT_ATT_RANGE:ANALYTICAL_ABILITY:450:950:1150:1250:1350:1550:2250] +
[MENT_ATT_RANGE:MEMORY:700:1200:1400:1500:1600:1800:2500] ++
[MENT_ATT_RANGE:CREATIVITY:700:1200:1400:1500:1600:1800:2500] ++
[MENT_ATT_RANGE:INTUITION:700:1200:1400:1500:1600:1800:2500] ++
[MENT_ATT_RANGE:FOCUS:150:600:800:900:1000:1100:1500] -
[MENT_ATT_RANGE:WILLPOWER:900:1200:1500:1750:2000:2250:3000] +++
[MENT_ATT_RANGE:PATIENCE:100:400:550:700:850:1000:1300] --
[MENT_ATT_RANGE:KINESTHETIC_SENSE:700:1200:1400:1500:1600:1800:2500] ++
[MENT_ATT_RANGE:SPATIAL_SENSE:150:600:800:900:1000:1100:1500] -
[MENT_ATT_RANGE:LINGUISTIC_ABILITY:1250:1500:1750:2000:2500:3000:5000]++++
[MENT_ATT_RANGE:MUSICALITY:700:1200:1400:1500:1600:1800:2500] ++
[MENT_ATT_RANGE:EMPATHY:900:1200:1500:1750:2000:2250:3000] +++
[MENT_ATT_RANGE:SOCIAL_AWARENESS:900:1200:1500:1750:2000:2250:3000] +++
Lamias have longer lifespans, but hit maturity slower...
[MAXAGE:180:240]
Lamia attack mainly through powerful tail slams... with high strength or training, these can really devastate:
[ATTACK:KICK:BODYPART:BY_CATEGORY:TAIL]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:tail slap:tail slaps]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[CASTE_PROFESSION_NAME:CRAFTSMAN:craftslamia:craftslamias]
[CASTE_PROFESSION_NAME:FISHERMAN:fisherlamia:fisherlamias]
[CASTE_PROFESSION_NAME:HAMMERMAN:hammerlamia:hammerlamias]
[CASTE_PROFESSION_NAME:SPEARMAN:spearlamia:spearlamias]
[CASTE_PROFESSION_NAME:CROSSBOWMAN:markslamia:markslamias]
[CASTE_PROFESSION_NAME:AXEMAN:axelamia:axelamias]
[CASTE_PROFESSION_NAME:SWORDSMAN:swordslamia:swordslamias]
[CASTE_PROFESSION_NAME:MACEMAN:macelamia:macelamias]
[CASTE_PROFESSION_NAME:PIKEMAN:pikelamia:pikelamias]
[CASTE_PROFESSION_NAME:BOWMAN:bowlamia:bowlamias]
Lamia "must be natural born leaders... because they sure don't understand how to follow."
[PERSONALITY:ACTIVITY_LEVEL:0:35:90]
[PERSONALITY:ALTRUISM:20:70:100]
[PERSONALITY:ADVENTUROUSNESS:0:40:100]
[PERSONALITY:ARTISTIC_INTEREST:10:65:100]
[PERSONALITY:ASSERTIVENESS:25:70:100]
[PERSONALITY:IMMODERATION:10:75:100]
[PERSONALITY:COOPERATION:15:45:80]
[PERSONALITY:DEPRESSION:0:30:90]
[PERSONALITY:DUTIFULNESS:0:30:90]
[PERSONALITY:EMOTIONALITY:10:65:100]
[PERSONALITY:EXCITEMENT_SEEKING:0:35:100]
[PERSONALITY:FRIENDLINESS:20:70:100]
[PERSONALITY:GREGARIOUSNESS:40:70:100]
[PERSONALITY:IMAGINATION:10:60:100]
[PERSONALITY:INTELLECTUAL_CURIOSITY:5:65:100]
[PERSONALITY:LIBERALISM:10:65:100]
[PERSONALITY:MODESTY:0:40:90]
[PERSONALITY:ORDERLINESS:0:60:100]
[PERSONALITY:SELF_CONSCIOUSNESS:0:25:80]
[PERSONALITY:SELF_DISCIPLINE:0:40:100]
[PERSONALITY:SELF_EFFICACY:25:70:100]
[PERSONALITY:STRAIGHTFORWARDNESS:0:40:90]
[PERSONALITY:SYMPATHY:15:70:100]
[PERSONALITY:VULNERABILITY:0:40:100]
No idea if you were curious about or cared about those trait-based numbers, but there they are for my vision of them...
EDIT:
Ah, and here's a great video clip I found...
http://www.youtube.com/watch?v=7TF7d4jvays&feature=fvstArtic snakes(!), and they are clearly using rectilinear motion. (You can see it start at 0:55 in the video, with the male viper moving across the rocks in rectilinear motion.)