How big of a potty area in yard?

for those that have a potty area, how big? I have 2 elderly dogs(17 this year), one is 8 lb and one is 32lb. their current area is about 6×7 feet. we were going to put a patio on one side of the yard and then expand it to their potty area once they pass but realized it'll be really hard to make the extension level. this is only for then to potty. they'll have freedom to roam the patio and the rest of our small yard. im trying to get the patio done asap so we can sit outside with them since one has arthritis and cant walk. the other is blind and deaf but loves the sun.