Welcome to TBN. Generally speaking - when using a rear blade, for whatever reason, weight will be your friend. The seven foot blade would probably be OK if all you were going to do is plow snow in your driveway. Recognizing that a seven foot blade is 22" beyond what the tractor specs recommend. However, if the blade will be used to create or maintain ditches then I would recommend the heaviest six foot you can find/afford. The six foot would put you back within 10" of factory recommended specs.
All this is based upon the assumption - if you go too big or too heavy or both, you definitely increase the chances of damage to the tractor.
I have found that my rear blade is not the best tool to maintain my driveway - in the summer. I have a heavy Land Pride rear blade but it still bounces along the driveway like a jumping jack. Its the perfect tool for winter snow plowing - but falls way short for summer gravel/dirt work.
I use a roll over box blade(ROBB) and a grading scraper for summer driveway maintenance. Both do an excellent job at smoothing & grading.
Just so you know - in the summer my gravel driveway becomes concrete. The wet spring causes it to set up and become almost impenetrable.