I used to enjoy med-sized game, but these days I always play with maximum conditions: biggest possible and largest possible.
15km x 15km gives a chance for me (and AI too) to press by maneuvering or flanking. With 50k, I (and AI) can try some serious combined arms stuff with multiple companies of armor and mech infantry.
Unfortunate thing is, due to 100 unit limit, I cannot field 50k-wise units in the field if I play early cold war or secondary echelon forces. If I play Italy or Poland, maximum practical limit for my forces will be 36k ~ 40k point due to unit number limitation. I wish AB loose the unit number limit to field more units, and allow biggest size map to 20km^2 or wider.
This ALL day, bigger is better. That being said, smaller scale engagements are very much still enjoyable. I love the scalability that can be attained with this engine, from small scale to large engagements with tons of maneuver. I wonder, theoretically, just how big of a map and engagement the engine could feasibly handle.