Game pixel art noob question

I don't know if it's the right community to ask these questions, sorry if it's not. I want to make a unity game with pixel art assets using Aseprite. And I have 2 questions that I have difficulty wrapping my head around.

1 - what is a good rule of thumb to know the resolution of my pixel art assets ? As I've seen around, it is highly recommended to have the same resolution for all pixel art assets in a game to keep consistently but should I base it around the amount of details I want for my game or are there other factors to keep in consideration ?

2 - does the "negative space" around my drawing matter ? How do I know how many pixels on each side I will keep empty ? Should I always have the same amount of empty pixels on each sides or does it not matter ?

I also know next to nothing about drawing in general so sorry if it's dumb questions but I'm willing to learn.