This sounds like a problem with scaling. There are two different places you can change things:
(1) in Parallels: Configuration…Graphics…Resolution. (If the machine is running, then View…Retina Resolution). There are three options here. On my setup I can’t really see a difference between “Scaled”, “Best for Retina display” or “Best for external displays” but for your setup maybe it would make a difference. You will probably need to shutdown Windows to see the change.
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If you don’t get satisfaction with the Parallels settings, you might have success with changing the scaling options for just Paratext:
(2) Right-click on the Paratext icon, click Properties…Compatibility…Change high DPI settings. I see two checkboxes and they’re both unchecked on my computer. Note also the link to “Open Advanced scaling settings.” I think these "Advanced scaling settings” get adjusted by Parallels automatically when you change the settings in (1)
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I hope you find the solution.