Hi! I think for most current animation you would be working at a 1920 x 1080 (pixels) ratio (a standard HD size in ToonBoom Harmony). So, if you were doing that on paper, say around 12.8 x 7.2 inches.
Really, the format and size is going to be dependent on what you’re making. These are just some guidelines. Ratios and sizes are constantly changing thanks to technology.
The very best online galleries of backgrounds and concepts are here:
Hope that helps!