Hi! Have you tried editing the settings? If you go into Add-ons, and click on the three dots beside Tabby and click on Preferences, it will open the settings page where you can set the scale of everything in the popup, and the size of the popup window itself. (the settings page is quite hidden right now, so I will fix that in the future release)

Edit: Tabby actually has a standalone settings page, which is why it isn't in one of the tabs. I created this guide with images on how to access the settings page. I do admit that it is hard to describe in words how to access the page. I hope this helps! https://github.com/Bill13579/tabby/wiki/How-to-Access-the-Settings-Page