You have several choices. Project does not "know" anything other than basic math. So if you say 1 painter can paint 100sf of wall in 1 day. You can:

1. Enter the task "Paint 400 sf of wall" and set the duration to 4 days.
2. Now assign painter 1 - Project will automatically calculate 32 hours of work based upon a 4 day duration task * 100% assignment units.

Now you realize you have two painters. The task "Paint 400sf of wall" task could be done by two painters -- the space is large enough and you have enough paint brushes. So, you ensure the task is effort driven -- this is in the Task Form at the bottom once you have split the screen.

3. You add Painter 2 to the task. Because the task is effort driven and you are adding another resource -- the total amount of work on the task (32 hours calculated from the first assignment) stays stable. Project drops the task duration to 2 days because now each resource can do 16 hours of work at the same time as the other resource does 16 hours of work.

There is no automatic way to tell Project that a painter can paint 100 sf of wall in 1 day, so 400sf should take 4 days.

I hope this helps.