Acrobat DC - Measuring tool

OK, so here's how you do it. Create a Measuring annotation and select it by clicking it with the mouse.

Then press Ctrl+J. A "console" window will open. Paste the following code into the lower part of it:

var spans = selectedAnnots[0].richContents;
spans[0].textSize = 20; // you can change the 20 part to the desired font size
selectedAnnots[0].richContents=spans;

Now select all of the code (with the mouse or keyboard) and press Ctrl+Enter.

It should edit the currently selected comment to the desired font size.

You can now right-click that comment and select "Make Current Properties Default".

Any new Measuring comment you will add from that point on will have this new font size by default.