Roe ears vary in length, sometimes you have long ones and sometimes surprisingly short ones. As a rule of thumb if you are looking at a buck that has antlers more than twice ear height, then you are looking at a very good one! Another good indicator is what the antlers look like when the buck is walking away from you. That view give you the best look at the main beams, if they have good mass and bulk then you are also looking at a good buck.
Finally the best bet as always is to go out and shoot a few, then gain your experience by comparing your view and assessment of the antlers "on the hoof" with the antlers in the hand.
Good luck
Lakey