So far with my adventures in !!SCIENCE!! aquifers I've found that the optimal size for dinning out aquifer tiles (without needing to dig the entire level as up/down stairs) appears to be 6x6. 5x5 is faster by a fair margin, but the greater utility of the floor space provided by 6x6 tends to be worth the extra time, if you are planning on using the space. 7x7 is feasible with some tactical smoothing, but the time difference between that and smaller areas is huge. 7x7 isn't really worth the extra effort unless you absolutely need that size. 8x8 was nearly impossible to do due to how much pushing was happening, so I wouldn't even bother.
So far I've just been digging u/d stairs around 6x6 blocks of earth from the base of the aquifer up to the top level, and then digging out those 6x6 blocks from the top going down.
It has the nice side-effect of producing modular 6x6 areas that have a usable floor (54% of mined space is usable floor, which is slightly more than using a checkerboard pattern, and a lot more than just digging u/d stairs everywhere) where just digging everything with stairs requires channeling and re-flooring if you want to use the space, which you pretty much have to do with a checkerboard pattern too. The downside is that it takes a lot longer to dig out the 6x6 grids than it does to just spam-dig stairs.
The positive upshoot of the 6x6 grid is that it pretty much turns the whole project into a grid of waterfalls, so your miners gain happiness like crazy. Once I was done digging out my practice area, all my miners were extremely ecstatic.