I just bought a 7.5 foot Howse HD 3 point mount disc this spring and it's been a good investment. we do about 5 acres.
I would add that it's not just about bigger acreage = bigger disc. You have to consider the access to your plots and how you are going to work them. If you have restricted access to the plots with narrow approaches or maybe the plots are in the woods, it's a totally different set of needs than just converting an old field to food plots where everything is clear and set up for big equipment access already.
Also, think about how you are going to turn at the end of each pass. A big, wide disc needs a bigger swing area to turn without really dragging the discs and risking damage making a hard corner with the discs down. This is where a 3 point comes in handy, just raise the disc, swing around, disc down and I'm right back to digging the next pass.
Doubling the size of your disc may or may not cut your discing time in half. Everything depends on how your plots are laid out and how efficiently you can work them with given size of disc.
Grouse