I think the top of my answer here may relate to your question. You’ll want to change fontSize in Settings. Although, it isn’t limited to the output font size.
UPDATE:
In JupyterLab version 2.1.2, you can just go Settings > JupyterLab theme > Increase Code Font Size several times, and it will increase the font sizes from both the code and code output.
Also see Python3 in JupyterLab: How to change font-size of the output from Python?