So unlike the army lists, my roman republican units, are standard size, the attack and defence values are easy to figure out, but what about the point value, should it be the same as carthageian spearmen, or the same as later legionary cohorts?
I can't tell you off the top of my head, I can't even remember if the points system used is in the main rulebook or the front of the army list. You'll be able to add the relevant points for the extra unit size. There won't be any additional points for the weapons or anything, just an additional point or two for the size.
There's a guide at the back of the army list book that provides basic guidelines for determining points values. It's very simple and straightforward to use, though I would encourage you to (with the permission of your opponents) tweak the resulting points values if you don't feel that they're appropriate for the games you're playing.