Welcome to TVPaint Community forums !

I found out how to shrink my drawings I scanned in by using the transform tool >> then scale. However, it would be nice if there were an easier way to do this, maybe a button on top?

How do you want to resize your project with an easier way than the transform tool in the main panel ?
Duplicate the function in the top bar would be completely useless and redundant, as it'd work exactly in the same way : choose the image and resize it

By the way, don't forget it's an animation software : it manages layers and instances on big durations, and any size modification could spoils the quality of your movie.