The best thing you can do is changing the display settings in Windows itself. This because that will change everything you see on the screen, not just one small window in one program...
Anyways, there is currently no way to enlarge the font size in Floating Contact windows (or also called Desktop Contacts).
Even not by changing the interface windows of Plus! itself (I just tried it, but the RichStaticControl element does not allow for a font size tag <= Patchou? *). This (advanced) method was the method I was going to suggest to you, but apparently this will not work either.
So, in the end, I'm afraid that this isn't possible, unless you actually alter the display settings in Windows itself.
* PS: I just talked to Patchou about this, he said he could add this. So, maybe in the next version it will be possible by altering the interface files. But still, the best solution is to change the display settings of Windows though...
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