convert string to list of bit

I'm looking for a way to read a file containing (HEX) numbers.

For each file I read, the width of the data is different (can be more than 32/64/128 bits).

So I'm looking for a way to convert the string I read from the file to a list of bit variable.

In order to make it easy, let say I know the size of the bits list (the data width) in advance.

Any ideas?