I'm targeting an Adsense campaign on a series of specific websites. How can I check the available banner sizes they have? Up to now, what I used to do is to used my browser developer tools (F12), or screencap the page, open it in Photoshop and check the size. But sometimes the sizes I find don't match any standard format: for example, I'm seeing now a 632x60 banner, and another one with 300x90, which don't match anything in Google's Display Ads formats.
All format size in Adsense is in fixed sizes that you can see at the link.
You forgot something that Adsense also has
responsive ads format (managed in google adsense if you log in) which has its flexibility work based on where the codes placed/installed. This format can be the same as fixed as I mention and also has its own size.
From official documentation -> guide to ad sizes
And also, if you want to do further reading - link
Google is great. It's all in documentation. If you are going out from formats, you're doing something wrong. Optimize your pictures to exact size you need and that's it. Don't put 2000x2000 picture up for a little banner and so on. Best solution is to put exact size picture.