@Karmin5 To maximise your earnings just go for the biggest place you can afford. But really what you should be looking for is to maximise your return on investment. This is way harder to work out but smaller places (and maybe multiple numbers of them) might give a higher ROI. It will totally depend on the specific market and what type of properties are in high demand.
In our case the ground floor comprises a 3 bed apartment the upstairs floor a 1 bed and a 2 bed apartment. The combined revenue from the top two exceeds that of the ground floor whilst the running costs of the two floors are similar.