Of course, smaller is easier. I just don't believe in small aquariums. I would never own, or recommend, anything smaller than a 40 gallon long for most species commonly found in an LFS. I fully realize the vast majority of people on this forum will disagree, so please, just don't bother.
I'll bother, simply because you told me not to!
It's all fine to make a strong statement and order people not to respond to it, but I'd like to read your logic. Lots of people believe in things, but discussions come from them sharing the thoughts that got them there. I'll give you some points to refute.
Bear in mind I generally don't keep standard LFS fish, but do have quite a number of common species.
I keep small killifish that aren't active swimmers and that breed well in single species tanks of 10 gallons. Many keep them in smaller tanks, but that would only be for egg collection in the short term for me. I prefer 20 gallons for them in groups, and tens for pairs.
I follow a self imposed rule that the tank I keep a fish in should have a front glass 8 to 10 times the total length of a full adult example of the species. By that rule, I won't keep any fish over 15cm because I don't consider it to have enough room to thrive in captivity. My largest aquarium is a 6 foot (72 inches/180cm), therefore, my largest fish will eventually reach 14cm adult total length. I stock lightly.
My largest tetras range at about 4cm.
Groups of fish from the Corydoras group do perfectly well in single species 30 inch tanks, often with a killie or a pencil to occupy the surface zone.
If you run community tanks or want larger fish, I can see cogent arguments against smaller tanks. However, in a world where space is expensive and many aquarists are fascinated by micro fish and shrimp, there's plenty of space for smaller tanks. I don't keep Cichlids, corys, or mid sized shoals of tetras in 10s - they are too small. Anything below 10 gallons is a temporary container to me. I like the 30 inch tanks I make for those fish, with 24 inch tanks, lightly stocked, as a minimum. I also have 40 longs, 75s, etc, for groups and communities, so it's possible I can understand your blanket statement.
Locally, my 2 functional lfs only sell microfish. It's an interesting trend. Smaller aquariums seem the future of the hobby, if indeed it has a future. So let's have some background for your strong statement,
@kwamjing What are the fish you keep? What motivates you to say we need at least 40 longs?