Custom Stave Size

I wish to change, for an entire full score, the stave size to 5.1. I thought that I knew how to do this. I haven’t been tinkering with that aspect of Dorico recently. I seem to recall, in page layout (under set up, I think) clicking on an arrow and the stave size decreased by increments of .1 at a time. The menu now presents a set series of stave sizes and then ‘custom’. I don’t seem able to get the custom option to do anything. What do I do next after selecting custom?

Many thanks in anticipation.

You’re in the wrong place! Go to Layout Options, then Page Setup, then fiddle with the Rastral Size or type a number into the Space Size box (they do the same job).

Actually, was in the right place, just not doing the right thing! Many thanks!