How tall is Canelo in reality? 5'9 Smith towered over him ...

Untrue. 5'11 is 180cm.

GGG isn't that tall, maybe with shoes he is but in reality he is closer to 175cm.

 

celebheights.com I believe reveals famous peoples real height, at least for the most part. With many people they have a guy who is 5ft 8 who meets up with them and poses to take a picture so their heights are unquestionable. Here they have:

Floyd - 5ft 7.25 (bout same as me)
Pacquiao - 5ft 5.5
Kell Brook - 5ft 8.5
Amir Khan - 5ft 8.25

 

Boxers pretty much always have their heights overestimated.
I.e. David Haye, who's closer to 6'1 than 6'3.
Fury who's closer to 6'7 than 6'9.
The list goes on...It's just to make them seem larger than life. But also, the time of day which you measure yourself will affect your height. When I wake up I'm 6'4 (193cm) on the dot, if I measure myself before I got to bed I'm 6'3.25 (191cm), we lose height throughout the day, and the taller you are the larger the loss is.

 

Yep.

I reckon Fury wakes up 6ft 8 or a little over but drops right down to 6ft 7 for most of the day. Me being a midget wakes up a little under 5ft 8 then drop to just above 5ft 7 before bed.

 

He's only slightly shorter than Martin Murray who is 5ft11

 

Khan is taller than Brook