6000 x 9000 is 20 x 30 inches at 300dpi. 30 inches is about the biggest i can make a file in photoshop without it getting laggy and misbehaving. Working on a bigger file is easier for me and it ends up making everything look cleaner. I’ve compared it before to drawing with a marker on a post-it vs drawing with the same marker on a big piece of poster board, you have more room to work with so you can be more precise.
To scale it down i just go to image > image size… (in photoshop) and set the width to 800 pixels keeping the proportions locked.
I don’t pay much attention to brush size. I think for sketching/lineart it’s around 30-40 pixels and for coloring I just change the size of the brush to whatever will wok best.
:o ………..yes