Setting UI fontsize
Via the regular preferences you are able to control the fontsize for the editor window, but the rest of the UI might be somewhat clunky.
You can manually edit the
ide.properties file located deep in
$HOME/.sqldeveloper folder (path depends on version). Please note you might have several.
Comment in the
Ide.FontSize and set the size.
Got the solution from an Ubuntu hint.
Found a Windows equivalent Googling.