Demand for unit size varies based on location. For example, 10' x 20' units and larger are in much higher demand at my facility (population of ~45,000 within a 15 mile radius), which is located in what I'd consider to be a more rural, but still city, community right on Lake Michigan. A lot of renters store classic cars, boats, 4 wheelers, snowmobiles, etc. and my 8'x10' units are much more difficult for me to rent.
Not to sound cliche, but location is everything in the self-storage industry. My facility is located in the middle of a residential area (rezoned residential lots) and I have nowhere near enough units to keep up with demand, but I know the facilities on the outskirts of town, which although they are mostly still full, have a harder time doing so and often have lower prices.
I'd imagine in the bigger cities maybe the smaller units are in higher demand due to renters storing more household items and less vehicles, boats and other toys, but maybe someone that owns a facility in a bigger city can comment on that.
The one thing I really dislike about my facility is that there is no additional land build more units, so when purchasing an existing facility keep that in mind. If it doesn't already have 125+ units, you may want to find something with room to expand.
In my experience income levels don't play a huge role, as I have tenants of all income levels. The exception of course being if your facility has 15'x20' units for $600/mo in a low income area, or something extreme like that.
Number of units is a tough one because that can vary from owner to owner. Do you plan to manage or will you hire that out? On-site manager? There are a lot of options for remote management of a facility, but if you can't physically get there every couple weeks at minimum, you may want management and would have obviously have to figure out at what number of units that would work.
Any other questions?