5-10 is much easier to manage and also allows for more precise manuvering etc. I find 8 is a good number for some reason but i don't know why
Besides, generally when the battalions are larger the creators of games make them take longer to build, i'd rather be able to get 3 groups of ten out in the same time as 1 group of 30. If my town is under attack i need guys quickly, not slowly.