I usually run units of 20. Enough to give good combat resolution, and in the limit of a point sink. I use them with full command and hand-weapons & shields. Overall, 270 pts. a pop
When I have saurus I like to use units of 20 as well. Some people favour a more MSU approach with smaller numbers, but I like the staying power of larger units personally
Yeah, I would suggest to take units of 20. IMO, the only practical use of Saurus is as an avil, so for that reason a full rank bonus going into combat will mean that you should be able to win or get close to a draw. With M 4, you can never be sure about getting the charge off, so I would suggest not going below 15 unless you intend to use them as flankers/rearguard.
Saurus are slow, but versitle. My anvil units are 20 with Full Command. Sometimes 25 if we are running really big games. I also use a number of US 10 Saurus infantry w/ musician units as flankers. They scare the heck out of people with good reason.