I use an Innate wallet, since it's slim, light and waterproof, I do a lot of cycling and I keep it with the fold on top so rain doesn't get in it if it rains.

If you know you have legs of differing lengths, be sure to get the bike professionally fit and you can get your shoe cleats adjusted properly so that you do not have one-sided knee / saddle sore problems due to a leg length discrepancy.