Become friends with a gear inch chart. You increase your gearing by either adding teeth on the chainring, subtracting teeth from the rear cog, or both. Your gearing is really a matter of the difference in size between the chainring and rear cog, not the size of any one thing. Also, a difference of one tooth on the rear cog is usually equivalent to 3-4 teeth on the chainring, so you can make bigger changes that way without having to worry about chain length as much. Set the chain length such that your wheel is close to the seat tube with your largest (as in most total teeth) gear combination so that you'll have room to let it out when you make changes.