In addition to having different sized Wallets, Beedle's shop also stocks extra wallets, which allow you to carry an extra 300 coins each.
After buying one, you can leave the shop and come back to buy another one.
How many Extra Wallets can you buy?
On a related note, what is the maximum number of Rupees you can carry at a time with the largest wallet and as many Extra Wallets you can carry?