I have the Veredus Olympic, the Equifit T-Boot originals and the Prestige leather (with leather lining). I find the legs stay coolest with the leather. The Veredus seem to be much cooler than the T-Boots.
As far as size, the T-boot and Veredus are the same height. Here are some not-great pictures to give an idea of size on a 17.2 horse (wearing an XL in the T-boot and an L in the Veredus). Not sure why the T-boots show up grey in the picture, they are all black.
I am looking for good support and cool (my horse gets crabby with hot legs). She's tall at 17.1 (Clyde x TB).
Any boot will always add heat compared to an unbooted leg - try each boot for fit; I agree that leather/leather lined boot seems to stay the coolest.
No boot will actually support the leg, protection against concussive shock is real, hard plate acts differently than a soft plate, do research that & decide which suits you better.