So, I have a simple question, but one I've been having problems with (on W7x64, using 2.9.7 Krita)

When I use Photoshop, it has one feature I really like: The brushes relate size to pen pressure by default. It also tends to not work as well and is missing a bunch of features I like, like image wrapping.

When I use Krita, however, all the brushes seem to relate Pen Pressure to Opacity. I have no idea what flow actually is. Whenever I try and edit the brush presets, I am not sure if I'm screwing up but I still cannot seem to make brushes work with pressure = size.

What should I do in this case?

take a good look at how fill_circle is structured. Other than that, the manual has some useful links:

https://docs.krita.org/Loading_and_Savi ... king_Brush
And for some step-by-step looks at generating brushes:
https://docs.krita.org/Category:Krita-Brush-tips