For decades Harold Carmichael was the only guy 6'4"+ that found legitimate NFL success at WR, & of course there were many many reasons as to why that was. As @Dahlly pointed out the lower end of the spectrum you find guys like AJ Green who was 211 lbs (& actually only 6'3" & 5/8's) or Ed McCaffrey who was 210-215 lbs. With frames that large those guys will naturally carry more weight, but I also think it speaks to their ability to be physical with DB's.it does seem like that is a magic number. I thought of julio jones but looked him up and he's 220. Most of the 6' 4 freak athletes are playing in the nba. So many tall guys just don't have the body control and fluidity.
Here are the sizes of the better big WR's as they entered the NFL.
Calvin Johnson: 6'5" 239 lbs
Brandon Marshall: 6'4.5" 229 lbs
Vincent Jackson: 6'4.75" 241 lbs
Plaxico Burress: 6'5.5" 231 lbs
Ed McCaffrey: 6'5 210 lbs
Cris Collinsworth: 6'5" 192 lbs
Harold Carmichael: 6'8" 225 lbs
Below are some of the guys who are currently playing. And truthfully the 6'4"+ WR has really emerged in the age of the spread offense & change in PI rules.
Mike Evans: 6'4.75" 225 lbs
Mike Williams: 6'4" 218 lbs
Michael Pittman: 6'4" 223 lbs
Chase Claypool: 6'4" 229 lbs
Allen Lazard: 6'4.75" 227 lbs