It depends on what the nano tank size is. I feel like it's more of a bimodal distribution, peaking at 30 gallons for nanos, then peaking again at 75+ tanks or tanks that are 6' long.
But from my understanding, 40 gallons is by no means a nano. That term's usually for tanks that are 10-30 gallons in size (pico being 1-10, and femto being anything less than 1 gallon).
Regardless, it's all about the planning. If you've got contingencies for what to do if something happens (like, an ammonia spike), then it's a lot easier since you know what to do.