At the moment it’s just Google fonts, though I hope and expect this will change with Blocs 3.
The dimensions of photos will vary according to your chosen site width and you may also wish to have different sizes for tablet or mobile, so there is no easy answer to that question. You have a lot of flexibility.
As for image dimensions, there are no specific requirements, but for backgrounds, I would suggest to use the images with the maximum width of 2500px, and don’t forget to optimize them using apps like ImageOptim.