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.
Agree with strewart. 6x3 seems to be the way. Also 6x2 works for me aswell, for more of a suport role. If you feild em 6x2 can be worth it to get 13. 13 being a magic number in this game. Have feilded em 5x3 also, but I prefer 6x3, more power in that formation.
I can concure with the above posters. I enjoy running my sarus units in one of two ways. Defensive 'anvil' style: Run these guys 5x4 with HW/SH only. They're slightly cheaper than spear sarus, have a better save and can last a long time while bogging down all but the most elite hard hitting troops. Offensive/Agressive style: I run this unit 6x3 w/ spears. Having 18 instead of 20 helps save some points and makes for 3 even ranks with some nasty attacks/strike backs. This can definitely be a good anvil unit but has a tendancy to evaporate more quickly because of less models and less of a save. The extra attacks are great though!
Occassionally I run them in a block of 30, six wide, and five deep... and put a hero in the unit. It's hard to pew the unit down to where it's not threatening, and at that point it can just roll over things that it hits. Mostly, it tends to bring a whole, whole lot of people's deployment arrayed against it.
30 with command and a hero with items is ~450-500 points in a m4 unit (with no ability to pewpew out loads of spells)... a large chunk of army easy to avoid or run around don't you think?
I would say that it is easy to avoid, but I don't see anyone who wouldn't want to try and set their hammers up to charge that, especially if they thought they had the advantage. Avoidable unless you have something equally as slow and expensive. Then plan on having your big expensive unit of sarus roll over what ever it is that the enemy is foolish enough to put in the way.