Hi everybody again.
It seems the issue is a bit deeper and depends on several settings at the same time which are scattered in several tools and Windows 10 settings. After I fixed the brush size issue, another one popped up: pressure sensitivity disappeared no matter which brush or setting I chose.
Nevertheless, I think I managed to fix my issue throughout all the apps, tools, programmes.
I will post my settings in case somebody runs into that issue and lands here thanks to the search engines.
Windows 10 with Affinity Photo 1.8.3.641 & Wacom Intuos (Driver version 6.3.39-1)
Windows 10: Windows Ink deactivated (via Win+R > "gpedit.msc" > Computer configuration > Administrative Templates > Windows Components > Windows Ink Workspace (double click > set to disabled)
Wacom driver: I have created an extra sub-setting for Affinity Photo. Under "Photo" > Projection > Turn off "Use Windows Ink" (Sorry, my driver language settings are in German)
Affinity Photo settings: Disable "Use Windows Ink for tablet input under "Preferrences > Tools".
I am curious what are your settings and if they work properly with your Wacom tablet.
Thank you!
Best regards
Emil