Collective term for data sizes (bytes, kilobytes, megabytes etc.)
Solution 1:
The capacity of a floppy disk is 1.44 megabytes.
The capacity of my flash drive is 16 gigabytes.
The throughput capacity of my internet connection is 50 gigabits per second.
(I would not change your fourth sentence.)
Solution 2:
Don't mix bits and bytes. The generic term for bytes is storage hence storage capacity.
Solution 3:
What do you think of "quantity of bytes"? The language in computer science is moving into an esoteric gibberish. No one tries to prune out all those nonsense and phantasy terms and use simple understandable names.