You do not need 45 deg. 72" blade will be plenty. I run about a 20 deg angel when I plow. How much does your tractor weigh? Weight ratio to equipment size is the key.
Chris
I found that for powdery snow on a brisk, cold day and in an area where I could drive quickly, 20 or 30 degrees worked just fine.
I also discovered that under these particular conditions:
1) very close quarters/no ability to operate at speed
2) wet, wet snow falling in air > 32 degrees
3) tractor with metal temperatures still below freezing due to low overnight temperatures
I had real trouble getting the snow to separate from the moldboard unless I cranked it all the way over to 45 degrees. Anything less and it would pile up in front of the blade.
If you decide to go with the 7' blade, make sure you can (easily) fit it thru your garage/barn door or wherever you keep your rig. First time I used mine I almost took some of the garage with me as I have to turn a little bit as I exit the door.
Also, I set my blade so it can float, both up and down as well as crown angle. My driveways are so wonky I have to. To avoid ripping up the grass part of one drive a chunk of rubber clamped between the cutting edge and moldboard works a little like a squeegee. Search the forums and you will find some interesting ideas on this theme.
-Jim