Lol. I did something similar. I ordered the 27cm Wodent and the Wobust, took one look at the 27cm Wodent and thought - he's too long for that - didn't even let him try it - so kept the Wobust! Coco kindly bought the 27cm one from me so it belongs to Mocho now

It looked nice in green. If I'm ever feeling flush I might get a new front panel and track in a different colour from Rodipet.
The 29cm Karlie wheel was big enough for Charlie - it was fine. But the 28cm cork one again just wasn't quite big enough for him to run without his head bent up slightly. So I reckon for big syrians like Nibble and Charlie 29 or 30cm is best, although I know the Trixie 28cm is supposed to be ok for some large Syrians - maybe the internal diameter is slightly bigger than 28cm or something.
I do kind of wish they made the Wobust wheel a bit less deep though (I think that's to fit rats in). The Karlie is 9cm deep which is about right - the Wobust is 13cm deep! Same size as the 30cm Silent spinner - that is also 13cm deep. I definitely preferred the Wobust to that though as the Silent spinner just didn't seem as well made and didn't spin well.