You will find that different things apply in different areas. When researching the area you want to buy into, this is one of the things to maybe ask some of the Property Managers.
For instance, near me there is predominantly small 3 bed homes, however 4 beds are a similar price (sometimes), & they command a higher rent as they are in demand. You might find that somewhere near a Uni there is a high demand for 1 bed units, yet near primary/high schools a demand for 3/4 bed houses.
This would be dependant on who you asked and their requirements at the time.
Eg, someone saving for a deposit on their own house may be prepared to rent a small unit costing them less each week so they can continue saving whereas someone else with a family that may or may not be saving will obviously require something larger.
Some folk are happy to rent and have no intention of owning (but still may have investments of their own) and will gladly pay top dollar to rent something considered luxurious because it would cost less to rent each week than to own.
I think this question is a bit ambiguous.... I mean in the city, one bedders may be relatively much cheaper to acquire, but return better rental..in the burbs, 3 bedder+ is a must and in the country, 2 bedders are often good rentals...