At the airports there's usually a frame-type thing that you can put your carry-on bag in to check that the measurements are within the size limits allowed (another stupid idea really - a bit late by then right? - if your bag is oversize, what can you do? this information should be printed on your ticket or somewhere).

In any event, the measurements on these frames give maximum dimensions for bags but again, if a cm or two bigger I don't think anyone would give you any problems.

I have a few bags to choose from and I've never had a problem with the following sizes:
- small wheel-on case 50cm X 38cm.
- wheel-on overnight bag (looks like a big handbag with wheels and an extending handle) 30cmX 51cm Although this one is longer than the other one, it's not as wide so once the pull handle is pushed down, it looks a lot smaller overall.

I think you'd be ok with the 50cm one, but not sure about 55cm.