The big question is not about the size of your properties. The big question is money and your drive to generate business. My advice is if you have the money/credit now, get the biggest best mower you can afford now. Then get the smaller mower later. I did the opposite. I got a 48" used machine to start off $2000, I out grew that in 4 months and had to turn around and get another machine and spent another $2000 on a used 52" In another 6 months I had to turn around and get another mower, since I had $4000 already tied up in mowers, I didn't have the cash to get a new one and at the time didn't want to finance one I got another used 52" for $4000. So I had $8000 tied up in machines in the first year. I should have just bought the brand new 60" I looked at for $6000. and then a used 48"/52" for my employee when/if I needed it. Because I had bought the smaller machines I was cranking the hour meters on them and by year 3 my $4000 52" had over 4000 hours on it and the $2000 52 had god knows on it. So I had to turn around and buy another one at the end of year 3 I learned my lesson and got a 61" brand new Which as it turns out could do the work of the 2 old 52's by itself since it was about 2 mph faster coupled with the larger deck size. I have about 80 lawns on contract and only 4 have places I can't run the 61" simply because of gates. 2 of the 4 are small enough to just do with the trimmer without a thought the other 2 can be done with the trimmer, but you don't really want too so I can get buy with only the 1 more if I had too.
However my story is probably different than most. This was my second lawn business and I was on a mission from day one to get 100 customers in my first year. I think I got to 120 or so I can't remember exactly I think I peaked in the low 130's just before the housing bust and I lost a ton to foreclosures.
Do I regret the way I went cheap? Not really, I still think it was a wise move, even though in the long run I paid a lot more, if I had not met my goal or only got a hand full of customers I only had the $2000 investment in that first machine.
As far as walkbehind or rider. I am a strong believer in the walkbehind. Hydro walk behinds are zero turns they may run a few MPH slower than the riders on paper but at least from my experience the overall time is roughly the same. The walk behinds have a few benifits that the mid mound riders and standers don't, first the out front deck allows you to get under and around things that the others can't and when going under trees with low limbs you can squat down behind the handlebars and not get smacked in the face. When dealing with hills, ditches, and ponds the walk behind cannot be beat the lower center of gravity allows you to mow just about any slope with out fear of rollover and even if it does roll over your feet are firmly on the ground as you watch the machine tumble down the hill instead of being crushed under it. 3rdly it is darn near impossible to get a walk behind stuck and if you do, it is light enough to get unstuck pretty easily. If you do your own repair work they are far cheaper to work on and the engines are usually cheaper as well since they can run a lower HP than the riders.
The down side. Tired legs but you get some great calf definition. lol