Also is it possible to set custom thumbnail size? Like 140x78.
Yes, via the backend not that easy as it uses sort of relative size value but via a custom theme you can set any custom size via function calls (this is actually nothing new and exists for years).
First I recommend to read the theming tutorial on https://www.zenphoto.org/news/theming-tutorial/ which also includes links to a bare bones theme.
The general documentation is available here https://docs.zenphoto.org (same as the in code documentation) and might require some PHP knowledge to understand. This is the most general function for custom images (there are more and sevaral ways): https://docs.zenphoto.org/function-printCustomSizedImage.html
Also of possinle interest:
And more on https://www.zenphoto.org/news/category/user-guide/