I think 10 inch is the most popular. The filters are easy to replace, but the choices are overwhelming because there are so many brands and sizes of micron filters, etc. Just remember if you replace the ro membrane, you have to make sure you match the gallons per day of your unit with the size of the membrane, otherwise you have to buy a new flow restrictor.
The flow restrictor has to match the membrane because it determines how fast the water passes through the canister. I think that is the only thing you need to worry about when changing sizes. The other filters are the same regardless of your ro/di gallons per day.