Are NP proofs limited to polynomial length?
The decision problem "Is the first bit of the input a 0?" can be solved in constant time and space - without reading the whole input.


Given that a Turing machine head moves right one step at a time, a Turing machine head can only read a polynomial amount of the proof in polynomial time.

While you could define proofs to exceed a polynomial length, only a polynomial prefix of the proof could be read in polynomial time, assuming the head starts at cell 0 and can move at max one cell to the right in one time unit.