@IrieMangaStudios
 
Going by the canon character models in the show…  
I’m pretty sure Blueblood is the largest (using the ‘huge canterlot unicorn’ model that we saw a few times during the Best Night Ever song) with frankly trunk-like legs and a comically oversized chest.  
Then Big Macintosh, though it can be hard to tell, but he has an unique intermediate size stallion model (aka large by Ponyville standards, but still smaller than many of the huge unicorns we saw in TBNE).  
Then Shining Armor, who, I think, has the usual pegasus guard model body minus the wings.
 
Admittedly, it would help if we had a direct comparison that could be made with show models. Rarity, Applejack and Twilight’s body types are all interchangeable, so it should not be hard to get a definitive comparative analysis using the mane six as a baseline.

Forgot one last thing, Big Mac has freckles. That is key.

@SquiggyBomb
 
I don’t mind bishies all that much, but there needs some variety and something, I don’t something to redefine the difference of a pretty boy and a handsome well-defined man.
 
Big Mac basically looks small and very young compared to the other two, those wispy arms does NOT sell the rugged farmboy look. Hair is NOT neat and pressed, it’s messy and wild.
 
Given that earth ponies are larger and physically stronger than unicorns, the image here doesn’t reflect that.
 
Shining Armor maybe a good user of magic but given he is royal captain, he should be physically imposing as well. He maybe the same size as Big Mac but slight smaller or shorter.
 
Blueblood finally could have a thinner frame maybe broader shoulder to give a more regal appearance but just not that big, he is a spoiled, high-strung pain. A smug look sells it more.
 
Those are my analysis on getting the characteristics for these three if they are humanized.

@MonkeyDzyrax  
Sorry about that. I just REALLY hate bishies.  
My bad.

@SquiggyBomb  
Complain to the artist not me

@MonkeyDzyrax  
But their facial structure is exactly the same. You shouldn’t have to rely on clothing and hair to make your characters visually distinct.

The one in the middle is Big Mac because hes holding a straw in his mouth. The one to the left is Shining and the one to the Right is Blueblood because of their hair

Who is who? Without color or something to tell the characteristic, it’s just a generic bunch of pretty boys.

@PonyPon  
Thank you generic bishounen design!
 

 
God I hate bishies.

I almost can’t tell which is which.