You may have a look at the New Platform Presets in JMP18
which will facilitate generation of plots with pre-defined settings.
Concerning font size of the grouping labels, there are some requests in the wish list to add a possibility to change the settings interactively:
function to delete subplot titles (or any GraphBuilderGroupBox in general)
Change Font (Size) on grouping variables in graph builder
Right-click to change Group font and font size
The last one collected some Kudos, so maybe it will be implemented in a future release ...
Unfortunately: At the moment, one needs to adjust the font settings in the preferences
... with collateral effects on all your Jmp windows
If you just need something like tiny / standard / big, you don't have to go to the preferences, you can use the one-click solution from Graph Builder Toolbar (which actually changes the settings in the preferences! - with the same collateral effects; just less effort)