# How many possible combinations of input files are possible using a 63-bit length?

I'm trying to calculate how many possible combinations of files there can be using a signed 64-bit file length, but can't seem to find a formula (or I'm using the wrong keywords). For example, the number of unique files that have a length of 256 bytes is $2^{256*8}$ = $2^{2048}$. I assume the total number of combinations across all file lengths will be some kind of geometric sequence.

The reason I want to calculate it, is because I'm curious how it relates to the number of possible SHA-256 hashes ($2^{256}$) of all these input files. Also, if I then calculate the SHA-256 hash of the SHA-256 hash of the file contents, how much have I reduced the entropy (if that's the correct term)?