If you have someone make them for you they can go as large as you won't as long as they have a table for it.
Screen Printed Transfers are made on Large sheets, and than cut down to the size you want. So, if you have two pieces of artwork, you make the transfer 11 x 17 then cut them in half and you have two. Actually the company that makes them cuts them for you.
If you're talking about ink jet or laser jet transfers, they come in different sizes too.
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