This is explained 4 posts above your question-
By default it only records in a 1 hour loop, deleting older footage as it goes- this takes up roughly 7.4 GB (4 cameras at roughly 1.8GB each for an hour of footage)
Only for sentry clips or manually saved footage does it keep anything else... each of those recordings is roughly 1.2 GB (4 cameras at 300MB each for 10 minutes of footage-- EDIT for sentry specifically it might grab the interior cam too, that'd make these 1.5GB each if so)
So you could hold roughly 80-100 sentry or saved clips in addition to the 1 hour recent loop on a 128GB stick.
Note it WILL delete the oldest sentry clips when it needs space and you are below 5% free space. It will NOT delete saved clips though so you can potentially totally fill the drive that way.
128GB is PLENTY unless you plan to record a ton of things manually and never clear the drive out.