When you say button accordion, do you mean CBA or do you mean smaller DBA?
CBA-wise, I know that bayan players used to play with one strap back in the days when the thumb was kept behind the keyboard at all times. The set-up is considered significantly inferior to double straps. With thumb-over technique I don't think it's possible to play like this at all.
Just don't.
DBA-wise, the story is pretty much the same I think, but because DBAs are still played with the thumb behind the keyboard (afaik), they are still sometimes used with a single strap. Since you can register your hand against the keyboard, and you don't need to have the same degree of precision as the CBA, I don't think you'll be too disadvantaged with a single, but you're probably still better off with a well set-up double.
In regard to the strap's size the exact optimal length for everyone is calculated as 2*(half its length). It needs to be comfortable on the pull, and you push against your right leg (and your right hand...although you're not supposed to) on the push.