How to increase the font size of 3 finger tap lookup results?
You can change it by modifying the /Library/Dictionaries/YourDefaultDictionary/Contents/DefaultStyle.css file.

You must change the part related to the html.apple_client-panel.


Open the file with TextEdit or any other editor.
Look for html.apple_client-panel body.


There you'll find this:

html.apple_client-panel body {
font-family:'Baskerville';
font-size: 14px;


This is the definition's font size.

If you want to change the size of all the fonts, you'll have to play around with the rest of the html.apple_client-panel related stuff.

When modifying the file, you'll be prompted to duplicate the document, as it is protected. Change it, save it, change (if necessary) its file format to .css, and just replace it inside the dictionary's package. This will require admin privileges, of course.

Note: Make sure you're editing the DefaultStyle.css file inside your defaults Dictionary package. Remember, too, to reopen the Application were you're testing this, or the changes won't be noticeable.