In my experience this is controlled by the plug-in settings. As I am sure you are aware on Scaler there are two settings in the Preferences tab: Font Size and View Size, which will size the plug-in window. Where plug-in suppliers have provided this option then this will be where you will have to resize the plug-ins, but this is not always available on some older plug-ins, notably from Air music.
For Reaper itself in the default theme there is a menu item LAYOUTS at the bottom of the OPTIONS menu where you can resize the fonts in teh Reaper panels.
This may vary if you choose another theme.
The biggest drawback moving from Ableton to Reaper is that Reaper does not support a Scene view, it does not have a Session view. However I believe there is a commercial plugin that for launching clips.
The big advantages that I think Reaper has over Ableton is its midi routing. mdid routing in Reaper is very flexible and easy. I’ve illustrated midi routing in the tutorial Basic Introduction to Multi Out.