Due to the size of the pitch, put them on a small a base as is possible - there is no "in game" reason to have bigger bases, as as far as the rules are concerned, every player, from a snotling to a treeman, takes one square, and one square only.
Some of the Big Guys, especially the older ones, will fit on a 25mm base, but most won't. Most will fit on 30mm bases from Impact Miniatures
The supplied 40mm bases are ok, but if you have to many on the pitch (ogre team), or a big ruck, the can prove to be a right pain.
You can get tokens to show whether BGs are stunned or prone (for example here
, but many of the independent BB fig makers produce similiar) which will make the use of BGs on the pitch easier, as most are to unwieldy to actually lay down on the pitch.