I may be mistaken, but I think Toady based the DF size unit is a body volume. Cubic centimeters perhaps?
Lets see then...
A human is 70000 size units. Assuming them to be 180cm tall, and assuming an avarage width of 40cm and a depth of 10cm, we get 72000 cubic cm. Seems about right.
Now, none of your values is a body volume, and not having a clue what a Bulette is, I cant tell how wide and tall it would be. One way to go about this is to make yourself an avarage human, and see how much his corpse weighs. Make an avarage human by disabling all the variation tags (remove their [ ), or just kill a whole bunch and take an avarage (its for science!).
The other way is to look up or guess the width and height of the creature and do the obvious math.
Chances are your weight and body volume arnt going to match, but you have a fair range of values tow ork with so that should be ok. If your realy set on getting them both right you can change material densities with something like
[SELECT_MATERIAL:MUSCLE]
[SOLID_DENSITY:(somevalue)]
I dont know what you mean by its 'reach', but melee attacks can allways only hit targets on ajachent squares, and ranged attacks like globs, firebreath, and trailing vapour have a set range.
And I just cant help saying this: Saying mithril should be silver is like saying Urist should be Gimli!