You can use the command webpinfo from google's libwebp package.
Example of a lossy webp:
webpinfo yellow_rose_lossy.webp | grep Format
Format: Lossy (1)
Example of a lossless webp:
The answer is very specific.
Target your final user.
I think you have a loooot of homework to do.
You could say that a standard number is 1080px, either vertical size on a horizontal monitor or width ...