How would I go about making all my digital art all same size in line art and subject matter cohesive in size

So recently I was looking through a bunch of artists
Instagram I really like that who make prints, keychains, sticker sheets pins etc.. and I was wondering how do they keep the line weight the same for all keychains/pins and main subject for prints roughly the same size. Like all their work looks like a cohesive big collection.

let's say I wanna do a series or set of anime characters with a theme how would I make sure they are all roughly the same size across all drawings I mean subject matter not canvas size and the same line weight.I hope I'm making sense haha

I made 2 chibi drawings for my friends I used same brushes same brush size yet one can out with thicker liner art even though all settings were the same how did this happen?

Hope someone can help lol thanks in advance haha