You can fit 4x generators in a mk2, which is what I am using as my base for the stacking generators. While a mk3 will let you squeeze something like 9 in a single blueprint, but you are really playing tetris at that point and it doesn't look natural. It's also only a little shorter than my 4x plus a 4x on top. Overall, I like the 4x in a mk2, as it scales more naturally.

I highly recommend building them via blueprint if you aren't already. The main benefit being that you can overclock them in the blueprint itself, and it will be built overclocked and with the power shards already in place.