What can I do to improve the size of csv file that can successfully be imported via Feeds and Commerce Feeds
The 403 HTTP code translates to access denied, which doesn't points to a problem in the size of the CSV file. Since it's using the batch API, which splits up the job in smaller chunks, it doesn't look like that it is the size of the file that is the problem. You should investigate the watchdog (dblog) and see if you can find the root cause.

This is the best answer I can provide with this limited information.