Most every answer you get on here will be 48. It seems as though most on here are of the "if it's bigger it is better mentality". You will even get people telling you not to get under a 52 or 60 because anything under those sizes are worthless. You will have people to tell you to forgo the walk and get a stand on. Stand on mowers are great but are more expensive than walk behinds. A walk behind can also have a sulky attached and even a 36 will pull around a good bit of weight.
We don't have enough info to answer your question as it stands. If the majority of lots you will service are fenced with gates keep in mind a 48 will not fit through many, if any, walk gates. A 36 will fit through the majority of walk gates except for the few that are 30 and under. Those extra small gates are few and far between in my area.
Keep the concept of time in your mind. Many don't want to talk about facts when you talk about mower sizes. If you take an acre of grass with no obstacles the 36 will cut it in 30min while the 48 will do it in 22. The time discrepancy gets larger as you mow larger areas and it gets smaller when you mow smaller areas. On a quarter acre lot the time difference would be approximately 2 minutes on mow time between the 36 and 48. If you get a 48 then any gated area you mow will have to be done with a 21. Any time you make up with the 48 is more than lost every time you take the 21 off the truck because the 21 does an acre every 105 minutes. If you were to do 10 accounts with the 48 as compared to the 36 you would be 20 minutes ahead but bring 1 lot where you have to use the 21 and the 20 minute cushion vanishes.
If you have no fenced areas and do not plan on taking any customers with fenced areas then the 48, or even bigger, is the choice. If on the other hand properties in your area are fenced then you should really consider the 36.