Have scoured the manual to no avail (inputting, index, sequence, enclosure adjustments all shown but not…) how can one increase the size of rehearsal marks?
I’ve looked in Engraving options and Layout options but it’s not there.
Have just read here:
Engrave > Font Styles
…but (i) this does not seem to have an effects and (ii) is it safe to change font size so universally in a project?
Finally found it. It’s under ‘Editing’ in the manual. 
One has to select ‘Rehearsal mark font’ from the ‘Edit Font Styles’ window:
Unfortunately this affects all Researsal mark sizes, in both full score and parts.
Is there a way to make this setting affect full score only? 
Make sure properties are set to “Locally,” select one rehearsal mark, use Select More to select all of them, and use the Custom Scale property to increase or decrease their size in the score layout only.
Thank you DanKreider! 
I changed my RM by going to
Step 1:Library>Font Styles>Rehearsal Mark Fonts, then set the font for inside the box / circle
Step 2: Cmd/ Shift/E >Rehearsal Marks > Enclosure, which has all of the padding etc
Yes, but the question was specifically to change one layout but not another. In version 4.2, rehearsal marks are a Font Styles property, which is global.
I’m using 4.2. Can’t see a ‘Font Style’ selector. I have ‘Parent’ and ‘Font Family’, and the font list on the left of the page that opens when I select’ Edit Font Styles’from the Engraver menu does not have Rehearsal Marks on it.
EDIT: It’s moved to Paragraph Styles