The height talk wrt Valdes is kind of a cliche to be honest. 183 cm is a decent height for a goalkeeper, and more than enough to handle corners or set pieces once he gets more comfortable with the physical nature of the game. Shay Given for one, was what 2cm taller than Valdes ? And he was consistently one of the better keepers in the league. David Ospina is a similar 183 cm, and he has been one of the best in the league through the latter half of the season.
Plus Valdes is an excellent athlete, with a real spring in his leap :
Hoek can help refine his cross-collection technique a bit, once he brushes off the rust and gets accustomed to opposition players ramming into him on every occasion, he'll be just fine. Sure, he'll be tested to his limits through the first couple of months of regular playing time (if it comes to that), but Valdes has always been a very scrappy and driven player mentally, so he won't exactly shirk from a new challenge.
The only worrying part is the state of his body, has he recovered fully ? And how he'll hold up (especially his knee) through the course of a rigorous 50+ match season including the Champions League, and the odd domestic cup appearance. A lot of really good keepers have regressed prematurely towards their early to mid 30s in recent seasons because of fitness related issues (Casillas, Cech, even Buffon when compared with their prime) so that's a bit of a concern.
If we don't sign a top replacement for De Gea and pen Valdes down as a starter, it'd be wise to target a quality young understudy who can develop slowly and step up into starting XI as and when required.