Yes, the bigger the better. The minimum recommended tank for Mbuna is 48" long (4 foot) by either 15, 18 or 24 inches high and or wide. This family do a lot of chasing about and some Mbuna genus can grow to 6 inches in length, not to mention that you do overcrowd Mbuna as a general rule.
If you plan on keeping the open swimming Haplochromis family then you should really be looking at a minimum tank size of 72" (6 foot). Another fact to bare in mind is that the smaller the tank the more frequent you need to do your water changes.