If you want to cover up your futon, first look for a futon cover that is designed to fit over the entire futon whether its in bed or sofa mode to protect it like a sheet. But for sleeping, you should be able to use standard size sheets for your futon. Many are full-sized although some are smaller or bigger. Make sure to check out the dimensions of the specific futon sofa you are looking for if you want to use specific sheets for it.