We had exactly the same problem with the pottys. We got him a little cheep on that we just had round the house, just introducing him to it. He hated sitting on it though, he too has long legs and he hated squatting..he looked so uncomfortable and would rather stick both feet in and balance
We got him a chair type one, and he seemed to take to that quite well, although he does only stand to pee now.
We have also got him one of these for the loo which is brilliant (they have different designs and quite good price)
http://cgi.ebay.co.uk/Child-Soft-Padded-Toilet-Seat-Potty-Training-Blue-Circ-/160485043583?pt=UK_Baby_Potty_Training&hash=item255da76d7f
He loves this as he see's us on the loo, so he feels all grown up.
But since all this - he has finally taken to his cheepy potty
sods law eh?!