Hi folks, I started to use Vivaldi recently and encountered the same problem (small font size in the note panel). Current solutions either require editing notes in the full-page note manager (not convenient since I want to write notes side by side to some opened pages) or zooming the entire UI (not ideal since I don't want the UI to waste spaces unnecessarily). This simple (but essential) issue was raised about three years ago, it's hard to believe it hasn't been resolved yet.
Here is my workaround:
(1) Enable the Vivaldi modding support following this guide. Don't be afraid! I assume you all run the latest Vivaldi (above 2.6), if so, the enabling process is very simple - just tick some setting options and select an arbitrary folder to hold the modding files (will be provided below).
(2) Save the below lines into a text file, and rename it to "note-mod.css" (or anything.css as you prefer, you only need to ensure the ".css" extension), you can also adjust the "font-size" (the higher, the larger):
.NotesEditor-Editor {
font-size: 18px;
}
(3) Put this .css file into the folder you specified in step (1), then restart Vivaldi, you will find the font size in the note panel changes!
NOTE: I tested this solution on the latest Vivaldi version (4.3) and it works well. You can also change the font itself besides size, w/ the following lines in the .css (this changes the font to "monospace", you can also choose your own):
.NotesEditor-Editor {
font-size: 18px;
font-family: monospace;
}