How do you change the default font size in Notes? Recently the view size changed?

Unfortunately, in a recent version upgrade the default Notes font appears to have been upgraded to Source Sans Pro 10 which as you say is too small. If you want to change this you have to venture into the dark arts of MindManager theme editing.

I find this whole area very confusing (especially as the Help isn't very helpful) and no doubt there are better ways to achieve this, but the following seems to work for me.

1. Create a new map in MindManager and then on the Design tab go to Edit Theme.
2. In Edit Theme click on Advanced Editor and then on Notes Theme. The Format Notes dialogue box should appear.
3. Select the default font, size and colour you want. If you stick with Source Sans Pro you might find as I did that 10 is obviously too small and 12 is a bit too large, but you can type in 11 even though it doesn't appear on the dropdown list.
4. Click on OK in the dialogue box then on Apply and Close in the Map Theme ribbon. This should return you to the map.
5. When you are back in the map the Design tab should still be showing. Click on the dropdown arrow next to the box displaying themes,
6. This should expand to show a range of themes. Ignore these but click on Make Current Map Theme the Default.
7. To test if this has worked, close the map you were using and create a new one. Open the Notes pane and see if the default font settings have changed to match the ones you set.

Edit: I should have made clear that I was describing how to change the default template notes theme which should then be applied to all new maps. However, on testing this seems to work only on a template-by-template basis.