change default font size in icloud notes

You can select text in iCloud notes created in Mac notes application , just right click the selected text and click on font > click on bigger ( you click on bigger more than one to create the text size bigger ) see the article Write and format notes on Mac - Apple Support

The other way is select the text , take cursor on top menu bar > Format > Font > show Fonts > move the scroll bar to increase the font size or you can click on desired size say 48 .