Two important things I think to note are:
I'm looking at the food and booze needs of a full fortress in vanilla, ie: 200 dwarfs. If roughly 10-15 farm plots of quarry bushes can support 200, I have no trouble seeing a 3x3 farm supporting a fortress. 3x3 is 9 tiles, so in one year that would produce 54 stacks of plants (9 tiles growing 6 each over the year), which if every stack has four plants, is 216 quarry bushes, which when processed yeilds 5 leaves each, comes out to 1080 leaves. That's more than enough to support a fortress of 50, or even 100, which needs just 400 food per year, or 800 respectively(according to the wiki, eating twice a season. If you divide how much food is produced, 1080, by how much a dwarf eats per year, 8, theoretically you could support 135 dwarfs on that small 3x3 plot growing QB's). So if you get your booze from traders or have another farm plot for booze production, you're all set. Which leads me to my next thing to note,which is:
I'm not even taking into account food from other sources, like fishing or butchering animals. If you have a small butchering operation going, I you'd need even less farm tiles.
Also, when growing quarry bushes, they can't be brewed into alcohol, so I'm looking at the bare minimum of food requirements. The dwarfs can drink water from wells or whatever, but I have no idea just how much slower they work, or if that has any effect on stack numbers or whatever. So with a 3x3 supporting 135 dwarfs, you would need to grow plump helmets during the winter and probably have another farm plot growing something else for brewing too. So 135 dwarfs over 4 seasons needing 4 drinks each, comes out to 2160 units of booze. Divide that by 5 for each plant and you'd need 432 plants. Divide that by 4 for the average stack of a legendary grower, and you'd need 108 stacks. Divide that by how many plump helmets can be grown in one year, 13, and you have 8.3 tiles. So tend a 3x3 farm growing plump helmets year-round, with a 3x3 farm growing quarry bushes year round, and you have the food and booze needs met for a fortress 135 strong.
So that's how I figure it out.
Also, none of these calculations take into account fertilizing, but it would be reasonably simple to do. Just change the stack size to whatever is common. If we take fertilization to simply double the stack size to 8, then that 3x3 would produce double the quarry bushes and double the plump helmets, leading to double the dwarfs living off of it. So with fertilization, two 3x3 for booze and food can support 270 dwarfs.