As a point of reference, Ohio State has 13 guys listed as WR, 7 of them are under 6'3.
Oklahoma has 17 WR listed on their roster, only 2 who are that 6'3 or bigger.
So I'd guess if D1 size is the criteria, and we of course consider OSU and OU as near the top of the pyramid that average WR size is closer to 5'11.