Squads of 5 are nice, especially if you intend to ballance what weapons that squad has. Giving each squad a crossbow dwarf can really help at killing enemies before they get to you, as can giving each squad a speardwarf and axedwarf (for melee organ peircing and limb-cutting, respectively)
The axe dwarf and speardwarf are IMO necessary in a squad (so the minimum size is 2; axedwarf and speardwarf) because without them, killing something may take too long if it's a zombie or a multi-limbed creature.
Zombies have no internal organs to peirce with a spear, but can have fragile limbs lopped off with an axe. Multi-limbed creatures usually have multiple legs to stand on, so cutting off one leg to try and drop their speed isn't useful, but stabbing them in the eye with a spear is...
Everything else can be killed by hammers, maces, and swords.
Also, unless you're fighting enemies that can be killed easilly, making a squad of just marksdwarves isn't a good idea outside of fortifications, because there's nothing to slow the enemy's advance towards your xbows. Also, because the spear and crossbow can both do peirce damage, they can be interchangable; an axedwarf and crossbow dwarf can work just as well as a axedwarf and speardwarf. BUT, the problem with the xbow is that is has ammo to deal with, and once that's gone, the xbow becomes a clubbing weapon -- No more organ peircing ability!
[ February 12, 2007: Message edited by: Mechanoid ]