Which is better to use on your horse, a small bit or a larger bit? I had heard that a smaller bit can cut the nerves in a horse's mouth but I don't know if that is the truth or not. I wouldn't want to use something that could hurt my horse.

Bits are a complex topic as they rely on a number of factors to determine how harsh they are. These include jointed or not, thickness, texture, length of bars, use with curb chain, etc. A plain jointed snaffle of a medium thickness is usually good for most horses. Thicker for a green horse or soft mouth, sometimes a bit thinner or a twisted for a harder mouthed horse if they cannot be taught to have softer mouth. A quick check of any tack catalog will show oodles of types from twisted wire, to long bars, to extra joints to do different things. Unless you're on a highly rained or competition horse its generally not encouraged to use such things without expert supervision.

Wow. I had no idea that there were so many different types and makes of bits. Before I buy one, I will look in a catalog and find out what the last owner was using, too.