What size (width / height / other) of the canvas should be in the chandelier so that the drawn character does not come out either too large or small?

Question

2D

This is my first time working in Illustrator, and it has happened twice already that I have drawn objects that are not in the normal size.
The first drawn one came out huge, in the unit, when you reduce the picture, the picture loses quality very much. Almost the same happened in the second case - the character came out tiny, with an increase, a blurry soap comes out.
What should be the ratio when creating a new drawing in illustrator so that these mistakes do not repeat?