Rest assured that there is no such thing as "too long a stay" ! I don't think you said where you are staying, but if it is an on-site resort you will find the trip easier and more relaxing. I think one of the best parts of the trip for my DH is putting away the car keys as soon as we arrive. He tries every way possible to NOT have to drive anywhere during our visit and is a wiz at the diney trans. sys. *Tyler would be proud of him.
We are actually going at the same time as you and if you read recent returnees postings and trip reports you will gather that the crowds, heat and thunderstorms this time of year can really wear you out and limit how much even the most devoted commandos can accomplish in a day (and evening, and e-night)....
If you can extend your stay to 7 or 8, by all means DO. You will be grateful to be able to take a "day off from the parks" every few days and just rest, rehydrate and enjoy your resort. this is really essential (at least to my family it is) when touring the world in the hottest months of summer.
* If you do plan to do this, make sure that you purchase Park Hopper Passes, not the length of stays or whatever they are called so you don't waste $$ on those non-park days. Unused days on Park Hoppers do not expire so if you don't use them on this visit, well, you have a good reason to come back sometime soon!
Have a magical (and safe) trip!
stacilee