How do I enlarge Mathematica's GUI font size?
Preferences - Interface - Global Magnification.
I have mine at 150 %
Edit: Sorry, I thought it was the Notebook text size.
You can also tweak your stylesheet if you just want some fonts bigger https://reference.wolfram.com/language/tutorial/WorkingWithStylesheets.html
Or Window > Magnification... but that changes everything including sliders and images
AHHH wait, I did not see your question, good question that needs to be addressed by Wolfram, that splash screen is not a notebook with an editable stylesheet. This is an accessibility bug I think. Please report to Wolfram
Very few GUI interface elements in Mathematica, but many have this issue. There is always an alternative way to access the same functionality without the GUI. I have disabled the Welcome Screen that you show by default and just use the standard menu operations since I do not like the Welcome Screen font sizes or colors.
The text should be using the ambient font and size as spec'd from the OS. What is returned from evaluating CurrentValue["SystemFonts"]? The output from CurrentValue["ConnectedDisplays"] might be useful as well.
Ian, you are so freaking cool, thank you for jumping in on this! Hopefully Tim is still exploring it since it looks like they had the trial version, but this is cool info and I am glad it is now documented here for others.