That depends on how much muscle YOU can supply. The larger the bolt cutter, the easier it is to use them, because the handles are levers. The longer the lever, the more force is applied per unit of effort.
I wouldn't willingly go less than 14 inches. My favourite is 36 inches.
Again, the smaller the bolt cutter, the more force YOU have to supply. If it is just a snip here and there, small is fine. If it is an entire fenceline, then larger is better.
I hope this makes sense. It's all a matter of the tool doing the work, and being really bulky, or you doing the work and the tool being cute and small. I want the tool to do the work...