What are optimal website widths?

I'm trying to figure out what the optimal sizes are for my website widths. I know they can be the standard 1024x768, but I'm reading online people suggesting 1366 for a width. Can you provide some suggestions for:

Desktop/Laptop width
Tablet width
Mobile width

Btw, I can't do %'s with the theme I have. It's gonna need to be in px. Thanks :slight_smile: !