What size shiki futon should I buy?

I am switching to a shiki futon as my bed but I am uncertain as to what size I should buy. The place I am buying from has converted their sizing chart into traditional american sizes ( twin, full, queen etc.) But I am uncertain what size I need. I have a full size western bed right now, but I use my bed pretty much like an office desk so I needed the space. But since the shiki is on the floor anyways, I am not sure if I should go smaller to save the room, or if I should stick to the bigger size...will I need the space?