I spent some time on calculating (roughly) horses/ponies’ weights and heights, and I’m willing to share them with you.
 
First of all, horse is not human, you all know that. I said it before, Flash’s height (from withers to hoof) was 180 centimeters, but I think it’s too big, cause considering Flash is a character in MLP. I changed it a little, his height is 155-157 centimeters, weight is over 550 kilograms (without wings, muscle is heavier than fat).
 
In my opinion, Flash is warm blooded horse, kind of like the offspring of draft horse(Percheron) and Arabian horse, I am always fascinated to Arabian horse’s elegant body shape and Percheron’s heavy muscle. He’s not a fast flyer like Rainbow Dash, but he runs pretty fast though, and has quick reaction.
 
If Flash became a character of my species(Equine), then his height would be 180 centimeters, weight would be 1200-1300 kilograms (with wings).  
I read many comments said Flash must be working out in the gym. Umm, the truth is no, he didn’t mean to do that, and I don’t attempt to draw Bulk Biceps although I do like FlutterBulk. I guess it may be his heavy armor, I calculated its weight, at least 120 kilograms(normally, Flash can carry more than 90 kilogram objects). Which means he wears that 120-kilogram armor in at least 8 hours every day (except Sunday), in training, patrolling, flying and fighting. This armor helps him build his muscles VERY WELL.
 
Generally, horse’s height is over 142 centimeters, pony’s height is under 142 centimeters. So if you say you are drawing a horse, for me, you are drawing a horse that height is 142+ centimeters and weight is 360+ kilograms. Please don’t mix up horse and pony.
 
Twilight’s height is 120 centimeters at most, and she’s a typical slim pony, weight is around 50 kilograms (without wings).
 
I know you may ask: since Flash is ten times heavier than Twilight, how can they produce offsprings? That’s simple, in fact, I can draw it in a picture, BUT it may involve with mature content, so I don’t do that. You can say, Flash lays down on his back, Twilight “sits” on the top. Or she lays down on the belly, rear up her butt, and let him “come in”. There are lots of “positions” they can figure out.
 
That’s my opinion/headcanon, I know yours may be different from mine.  
(P.S. The reason why I don’t like drawing feathers on his neck and chest is because it reminds me of chickens, I want him look like a real muscular stallion, not a male chicken/cock.)