Orbea Onix Sizing?!

jenz999
jenz999 Posts: 2
edited January 2014 in Road buying advice
So I currently have an Orbea Asphalt in a 51cm and am 172cm (5'7). I've been looking at an Orbea Onix T105 2013 in a 54cm... I do feel my 51cm is slightly too small but would the 54 be too big??
Unfortunately no where in the vicinity stock Orbea so I can't go and test!

Any advice would be very much appreciated!