Yellowstone = 3,472 square miles. Grand Teton = 485 square miles. Depending on how much hiking you plan to do, etc., you may want to consider a little more time in Yellowstone vs. Grand Teton. There is plenty to see in both parks, however, so no matter how you split your time, you won’t see it all. If you’re going to be mostly sightseeing near the roads, however, you may want to plan for a little more time in Yellowstone.
Delaying your trip is a good suggestion. Both parks are spectacular.