CLaPorte has answered well, and I will elaborate a bit.
It has been widely proven that people with horse allergies can be around Curlies. I have never heard otherwise, and have heard many stories of people whose significant others or children have allergies and they switched to Curlies.
Being out crossed they do very greatly, from mini to draft. My two being draftier bred, but still only 15.1 hands. But at that height they could easily carry a heavier rider, as the breed's short back and dense bones make it a lot more solid. You will find a lot more Curlies that are 'normal' horse size from 14.2 - 16 hands, and horses shorter or taller than that are around, just a bit harder to find. There is a breeder that I know of who has the very drafty Curly stallion BNC Big Ben, and he is about the size of an average Belgian. He has quite a few foals on the ground this year, and they are huge. The breeder's son is allergic to horses, and there are countless pictures with him in close proximity to the horses with no reaction. There is no reason someone 6'3" couldn't ride a smaller Curly, they just might look a bit awkward. But with the density of their bone, even with a taller rider they'd still look like a big horse.
Hope that helps. :)