In terms of cubes, 4x4x4 (exterior dimensions) is slightly too small, 6x6x6 is too big, and 5x5x5 is plenty big enough. With 5x5x5, you get 3x3x3 interior, which is enough for 27 pattern providers + CPUs, which should be plenty, ex: 12 patterns + 15 CPUs or 10 patterns, 17 CPUs, etc.,. You can always change the ratio or even expand your MAC later if need be.
Unless you encode *everything* from NEI into your MAC (and end up not using most of it), I have a hard time imagining you will need more than 10 to 12 pages of patterns. And, as discussed earlier in this thread, there's not much point in spamming CPUs beyond a certain point.
In terms of hard lmiits, the only MAC size limit I've seen is power. You *can* build a MAC that requires too much power to boot up, seriously.