I'm using a panel with 39 pixels height and I added every plugin I could find that shows icons. I see 3 different behaviors: 1. Free Space Checker completely ignores this setting and the icon size remains constant (looks like 32 pixels but I'm not sure). 2. Window Buttons shows 16 pixel icons for any setting between 1 and 31, and 32 pixel icons for any setting of 32 and above or zero. 3. Icons on all other plugins change with every change on this setting, as expected. I also noticed that with the icon theme I usually use, Oranchelo-Beka, the icon for the Whisker Menu changes if I raise the size to above 48, but it didn't happen after I switched to Adwaita icons. Bug in the icon theme, I guess. Xubuntu 19.10, upgraded from 19.04 with xfce testing repo enabled.
Unfortunately all of this is expected. 1) Please file a bugreport with the free space checker plugin, it doesn't support the new panel API for setting the icon size yet. 2) The window buttons plugin is a little special, unfortunately it has only those two options (the underlying external library only offers 16 or 32px icons...). 3) Your icon theme has different icons for different sizes, so yeah, it's an icon theme bug.