No, the number of characters doesn't matter (after a certain point). You could generate 200,000 characters and that wouldn't be any more random or less predictable than if you generated 100 characters because you only have (at most) 2^32 possible seed values and each seed value will produce the exact same sequence, no matter how long you make it.

Update: To be more precise, if selecting from a set of 64 characters, then using 6 characters gives you 2^36 possible strings. So using more than 6 characters is useless if you have only 2^32 possible seed values.

        - tye (but my friends call me "Tye")