P: I am not able to define the size of my brush with my wacom pen

You just saved my all life! Thanks we have to report it to Adobe and share it during the time they correct it!
I only had to uncheck "Use Windows Ink" for my Wacom Cintiq 27QHD. It workw again so many thanks.

Thank you SO SO SO much !!!

Ok I found a solution for Photoshop. Create a file named  "PSUserConfig.txt" and this file add the line "UseSystemStylus 0" (without the quotes). Then copy this file to 
C:\Users\<your username>\AppData\Roaming\Adobe\Adobe Photoshop CC 2018\Adobe Photoshop CC 2018 Settings. 

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Finally in the Wacom drivers create a profile for Photoshop (if you haven't already) and then go to "Mappings" and uncheck the "Use Windows Ink" checkbox. 



These changes will make PhotoShop use a different API for accessing the stylus (WinTab instead of Windows Ink) and with these changes I managed to get both pressure sensitivity and my pen+ALT functionality restored

Yep thats just what i did. reinstalled the driver but it only drives me crazy 😞

I have got the same problem. When i click the pen button the brush size selector briefly pops up but almost instantly disappears. I already tried reinstalling/resettting the pen driver, but no joy.