When I sketch, I usually use around a 5-9 px brush with 100% hardness and opacity. When I do line art, I usually use a 3 px brush with 100% hardness and opacity. When I color, the brush varies greatly depending on the size of the the area I'm coloring, but I usually always keep it at 100% hardness and opacity as well. When I'm shading, the brush size also varies again depending on what I'm shading, but I often vary the hardness and opacity of the brush at that point too.
But the sizes I use are not standard in any way... It also depends on the size of the canvas you're working on. Working on a 1000x1000 pixel canvas would work just fine with a 3 px brush size for line art, but working on a 3000x3000 pixel canvas you might need to work with a 9 px brush or something.
It's also not the only way to do it. Sometimes I switch it up and use different settings just to see what happens. Mess around with it and find out what works for you.