Depends on how long you want it to last. You can just paint right over the rust which won't last long at all, or use abrasion (tiny piece of sandpaper, fiberglass pen like shown above, or easier might be a dremel tool) to only remove the visible rust, then put paint on to seal the edges of existing paint, OR if you want it to last long term then you need to enlarge the area till you are at clean metal under the edges of the existing paint.
It is difficult to match metallic paint with a touch up pen type product, even what's supposed to match can end up looking lighter or darker so there is an argument to be made for keeping the paint patch as small as possible then looking at it later to see if it needs redone. Make it too big and you could end up needing the whole section professionally painted to look right.