There is an option in TVPaint's Preferences that multiplies the UI size--try dialing it up to 2. Since that seems easiest, I would try it first. If that doesn't do what you need, reset the multiplier to 1 and try the following procedure.
Assuming you're running TVPaint on
Windows 10, do this: First, right-click on the program icon and open its properties panel. You can do this on a shortcut. Click the Compatibility tab and click Change High DPI Settings. In the window that pops up, enable Override High DPI Scaling Behavior, and then select System (Enhanced) in the list. I don't find it necessary to change the scaling in TVPaint's preferences but in Window's Display Settings, I have Scale And Layout set to 200%. The System (Enhanced) setting works well with TVPaint for me, but there are 2 other options on the list you can try. I found that a few other programs that also have scaling issues run better with the 'System' option selected (i.e., not 'System (Enhanced)'.)
Here's an example of what TVPaint looks like on my laptop computer using these suggested settings.
My laptop has a 4k screen, so without these settings, the UI looks similar to your example: very tiny.
I guess if your Windows 10 scaling is set to 100% and you don't wish to change that, you can probably combine the above with the UI scale setting in TVPaint's preferences.
If you're using an
older version of Windows, here's another procedure you can try:
http://forum.tvpaint.com/viewtopic.php?f=10&t=10388
If this is happening on a
Mac, hopefully somebody with a Mac will pop in with advice.