I paint small paintings until I get the brushmaster challenge - that's 30 paintings done. Then they paint much faster. Not only that, the number of paintings you do increases the value of all your future paintings. So, if you are aiming for very valuable paintings for a dynasty, then you should paint small paintings all the time, until you paint a portrait of someone, and then paint them as big as possible for the portrait. Also, since small paintings are much faster, you should achieve the other challenges faster, making your paintings more valuable, and giving you more chances to paint brillant and masterpieces.
Just to be clear, you have someone else to paint the portrait of your founder, right? Because you can't do self-portraits (unless they've added something in a patch that I'm unaware of).