Well, the Paint tool uses a "default" 3ds Max's brush - just like the Paint Deformation tool (where you can find settings for this brush under the Brush Options button). Did you try to right-click over the minimized window and choose Restore?
If that doesn't work, I would suggest you to "reset" 3ds max by deleting your User files. To remove the user profile:
1) Ensure that 3ds Max/3ds Max Design is not running.
2) Navigate to the following path (Windows7/8).
C:\Users\%username%\AppData\Local\Autodesk\3dsMax\2017 - 64bit\ENU
*modify the version and architecture to suit the case
3) Rename the ENU folder to ENU_Old.
When 3ds max is launched a new ENU folder will be created. If the issue has not been resolved, users can revert to the previous settings by deleting the new ENU folder and renaming the old one back to what it was.
Hope that helps.