Unit size is an interesting question, and quite often depends on what you are paying for in a model. What do you want the unit to do? Why does it cost what it does? Skaven slaves are only a few points, really useless on their own, but a large unit costs basically nothing so it is best to give them a huge unit - then they can outnumber, have ranks and afford to lose quite a few before their static combat res starts to disappear. On the other hand, HE swordmasters cost a fair bit more points per model, they are absolute killing machines with 2 s5 attacks at WS 6 always striking first, you pay all the points for the killing power so you damn well want as many models as humanly possible in contact when you charge, and given how much they kill all the extra points required for 1 extra rank is simply not worth it.
Where do saurus fit on this spectra? They are a fairly high points cost per model, they are a bit slow, but very hard to kill due to good armour and high toughness. They also dish out some serious pain on the attack with multiple attacks each and a full two ranks fighting when they are not charging - this is very powerful. Add cold blooded to this, and they have pretty decent holding power as well.
Will 10 of them stand up to most infantry units? Maybe, probably not. It will also only take one or two kills to render the unit a lot less effective. How about 15? Now they are getting up there with two ranks, able to absorb a good amount of pain and able to dish out a fair bit of their own. I personally like 18 - 6x3, then they get even more attacks and are cheaper than the 20 strong unit.
They are a pretty versatile unit though, so they will really work in a number of roles (support or frontal assault) and at a number of unit sizes. I will say that I think 25 is too high, you are paying a lot for the unit that is only about as good at 20 or even 18.
Wow I sure crap on once I start typing don't I.
