It's surprising but Roach will grow big in small waters. I was fishing a small irrigation pond back in 2000 that had been stocked with roach and carp in the 70's, using bread flake as a hook bait and feeding bread mash right under my feet next to a small jut of reed, i was catching roach after roach all about 5oz in size when all of a sudden I thought i'd hooked a carp. I could tell by the fight that it wasn't a carp and this is what was landed.
It's not a brilliant picture because I only had my phone but the box is 29cm long. I caught that roach followed by an almost identical fish, both over 2lb from this pool.
You should be able to tell from the photo how big it is, I'm at the far end but there was some really nice roach in there.