Can you change H/W Buffer Size from within PT without the need to quit PT?
No. Need to use their Control Panel and shut down PT.
Which audio interface did you use for these questions?
Native Instruments Komplete Audio 6
Which OS and version, PT version, etc.
Windows 10. Pro Tools HD 12.5
See it in video form here
KOMPLETE AUDIO 6 - UNBOX, SETUP, RECORD
(look in Description and go to the Using with PT segment)