Yes, they can work in any size tank, but as you get larger you would need more than one and it would cause some odd flow patterns for the water and lots of noise.
With a canister you can get even water flow in a left to right direction (or right to left) to simulate a slow moving river. They are also far quieter, and for that reason alone I'd use a canister on anything 40g or higher. I really, really, do not like the noise air pumps make!
You can set it up to provide flow on one side and less on the other this way as well. Definitely quieter than air.
As a side note with live plants, there is the argument that will never be settled on if in a natural setup (no CO2 injection) that surface disturbance will affect the scarce levels of CO2 available for the plants. But lots of people use them (myself included) on small tanks with no ill effects so /shrug
If CO2 is stable in the just water, bubbling or surface agitation cannot raise or lower the concentration, that is the equilibrium level based on air / water exchange.
Add only plants that use up CO2, bubbling or agitating can only replenish the CO2 back to the equilibrium.
Add only fish and decaying material that will add CO2, bubbling and agitation can only reduce the CO2 down to the equilibrium.
Add both fish, plants, decaying material and bubbling and surface agitation and the only place that the CO2 can end up is back at equilibrium.
As long as the plants are good with that balanced level of CO2 it cannot be hurt to disturb the surface at all.
Jeff.
