Clearly since Tolkien never had the time to make his work consistent (and what would we all do with our time if he had?!!!! ) there isn't going to be a definitive answer. I think it says somewhere (UT?) that the reason that old Gondor had little in the way of cavalry was due to the fact that few horses were substantial enough to carry a Numenorean. British guardsmen were traditionally sixfooters ... much taller than that and the riders are practically going to need rollerskates since few horses are over 18 hands (6ft at the withers) - if hte H&H website is typical - only 4 horses for sale over that height compared with 460 in the 16-18 hand range. That must have been quite a palfrey galadriel had!
Of course it is possible that they had larger horses but it seems unlikely. Based on armour destriers were no bigger than 16 hands though powerful..
Edit - of course Elves would not it seems be anything like as heavy as even a lean human of hte same height but they would still have very long limbs - would look like the tall skinny Thelwells....
like this!!
http://www.thelwell.org.uk/images/po...s/Ponies11.htm