Agility, aggression, attitude, all count. Plenty of tall keepers are shit on crosses, just as plenty of tall centre backs are.
Tim Howard wasnt imposing, he is 6ft 3, but was often rooted to his line, especially later in his career, I used to bemoan him not coming for balls in the 6 yard area
But we had Fellaini who was a giant and good on crosses, Distin, Jagielka too. And we had Tim Cahill, a dwarf, who was superb defensively on crosses.
Bernard, Walcott, Schneiderlin, Delph, Davies, none of them attack the ball in the air, in any area of the pitch, the way Fellaini, Cahill and even Osman did.
I'm not sure conceding set piece goals is improved dramatically just by having a taller keeper, when we only have 3 outfield players who attack the ball in the air.