Bikes don't run small or big , they are whatever size they are. The problem is that many manufacturers use different arbitrary methods to size their bikes. The Obrea sizing chart that I'm looking at for that bike says 53cm is good for 5'8 - 5'10 or 173cm to 179cm so not sure what you are looking at that says the 51 would be the right size for you. This Orbea's size 53 is what most other manufacturers would call a 54 or 55 based on the top tube of 549.1mm. The top tube is the most important part of fit on a road bike because it's the important part that is the least adjustable without effecting the bike's handling. I'm about 5'10.5" and 32" inseam and I prefer a bike with a 545-550mm top tube. The one thing that's curious to me is that they put 170mm cranks on the 53 where most companies move to 172 cranks when they get to 54cm bikes which is what this bike really is. But since you have a shorter inseam, the shorter cranks would probably work fine (and really 2mm is a small difference). I think the 51 with 536.5mm top tube would be way too small for you unless you have a very short torso. Since the 53 only has a 490mm seat tube there should be no issue getting the saddle low enough for you to comfortably reach the pedals. If you can't test ride any bikes to compare it to, I'd at least use this fit calculator which is really good and will get you in the ballpark:

https://www.competitivecyclist.com/S...ulatorBike.jsp