If you want to carry a large balance on a credit card, hat features should you look for?

You cant "carry" a large amount on a credit card, as a credit card is for "borrowing money" and then paying it back at a later date. If you wanted to spend a large amount on a credit card, you would want to get a credit card with a high limit, meaning that you can spend more before you have to pay, as well as a low interest rate or an interest free period, so that you would pay closest to the money you previously spent instead of paying more in the form of interest.