With PCoIP, certain settings are set by the remote client; examples would be timezone and screen resolution. That's why, say, screen resolution is greyed out.
In your case, the remote client is your P25. The dpi setting for font size is also set by the remote client, but obviously zero clients don't have that setting. You can set it manually by making a registry change on the vm. In my case, I made a gpo with a custom registry change that sets \hkcu\control panel\desktop\logpixels to 120. You'l also need to make sure this runs in the user's context for it to work.
I think there's a limited list of numbers you can set logpixels to. If 120 doesn't work, I'd search technet/google for other values.
