[2749] Add comment that we know there's room available in the buffer
[2749] Replace size() calls with literal values

This gives us a very tiny speedup, as size() was a const method already.
[2749] Remove space between sizeof and (
[2749] Update length argument
[2749] Update name of test (fix case)
[2749] Remove redundant checks
[2749] Update resize() call arguments to be sizeof instead of literal values
[2749] Minor comment change.