For larger battles, Fire & Fury works well (there's some Mex-Am adaptations on their website) though I've not tried the Regimental version which would lend itself better to the size of most battles.
Spring 2015 also sees the release of 'Gone to see the Elephant' by TVAG, which are period-specific rules, with a lot of supplementary information, worth looking out for.
As previous posts have mentioned, SP is excellent for games with up to 150 figures per side (and can handle more with a few tweaks to the way the Action Deck is used) at a 1:5 or 1:10 scale, which covers most of the battles.
Which figures are you going to use? I keep being tempted by the Eureka 18mm range (some more packs being added soon, I hear) but Blue Moon have a small range in the same scale, and the First Corps 28mm range is very characterful.