Is it possible to change the default size of the preferences popup window?
AFAIK no, but you can change any area's type to user preferences via the menu in the far left of the header:
You can then temporarily make it the size of the entire blender window by pressing
⎈ Ctrl↑ Up arrow. Toggle back to the previous screen layout with ⎈ Ctrl↑ Up arrow again.
(⎈ Ctrl↓ Down arrow works too)
If you just want to get to the prefs quickly with more screen real-estate, then perhaps this might help.