I too have Nigerian Dwarfs, and I own my bucks for the same reasons you listed. Bucks get a bad rap over their extensive use of cologne during rut. My little guys only go into the fragrant state during fall and winter. Yes they can breed year around, but the don't anoint themselves year around.
Nevertheless the boys are at the back of the compound.
Since you have just one buck and his buddy, your starting point of the 8x8 covered area and 4x8 sleeping area (which honestly they probably won't sleep in unless it is raining) give you 96 square feet, and your question being how much more space would they need... yeah sometimes I tend to state the obvious, anyway, so figuring on 130 square feet for one, and say another 70 for the second plus you 96 = 296 square feet.
Now, beat that against the shape of your area, mark it off and stare at it. Does it look reasonable to you, not me, but to you? If you think your buck and his buddy would be happy there then go for it.
Lord love a duck, they are your animals, it is your backyard and money, and it truly sounds like your intentions are for the best you can give your animals. Heck, my rabbits are 30"x24" cages... lots of discussion can be had over is that enough space. Would the rabbits like more space? Sure, wouldn't you like to have a larger house. Is it practical... probably not.
Onyx we all do what we can for our animals. Some can do more, some do less. What is more important is how you take care of them. If your boys are getting rowdy with the fenceline then look to see if they need new toys, or if the girls are flirting with them. They should normally be happy campers with toys, food, and your attention.
Good luck, keep us posted, and pictures once your are all settled would be lovely, please and thank you.