A PRNG with a 32 bit seed can only generate up to 2^32 different sequences of 32bit numbers, which are guaranteed to repeat after 2^32 generated numbers. Without doing the math, Even for a 3mb file chances are very slim there'll be a seed that generates the entire file. The number of unique 3mb files is astronomically larger than the number of sequences you can generate, even if you'd try every possible substring of all the sequences. So you need a PRNG that can generate more sequences, which you can only achieve by making the seed larger. What the person you were replying to was implying is that the size of the seed might approach the size of the sequence you're trying to generate.