Allow specifying max size for the preview

[Goosegit11]

Is your feature request related to a problem? Please describe.
I always get frustrated when the preview is too big. I want to control its size from the addon settings.

Describe alternatives you've considered.
The Imagus mod has this feature, it allows you to specify a maximum width and height:

Additional context
I tried to do this via the enlarge zoom factor option, with set it to 0.5, but it doesn't accept decimal numbers.

Describe the solution you'd like.

I prefer the third one, but you can come up with your own way as well.

1. Do it the Imagus mod way, introducing max width and height.
2. Allow set factor to decimals and numbers below 1.
3. Introduce some kind of percentage. Like, set it to 75% and preview will be 75% of the original size.