Upon further research, I have determined that for this specific application, a solenoid valve designed specifically for a gravity fed systems is needed. I found that the valve here should work for the given application. The pressure rating for this type of valve gives the maximum container height allowed where the valve can still open and close properly (in this case 7 ft). Although there are a number of different factors which would affect the choice of valve:
There are numerous different types of valves, but the valve found at the above link is a direct-drive type valve where the solenoid directly moves the valve plunger. Other valve types operate differently, for example pilot operated valves utilize the line pressure for operation instead of relying solely on the solenoid. Another important point is whether the valve is to be normally opened, or normally closed (what state the valve is in when it is not energized). My understanding is that choice of valve type depends on the fluid in the system and the pressures involved.
The pressure rating, in this case (gravity-fed system), gives the height of fluid that the valve can retain before it fails to operate properly. However, some valves also require a minimum pressure to open. In the case of the gravity-fed system, the pressure is dependent only on the height of the fluid in the container before the valve, not on the width or other dimensions of the container (Hydrostatic Paradox).
Different fluid types generally require different valve materials. For example, based on this document (page 526), sodium hydroxide requires a stainless steel valve assembly. It is also necessary to consider the temperature of the fluid in question, as that will also affect which valve material is needed.
Actuation speed depends on valve size and type, with smaller valves operating faster than larger ones. Solenoid valves that utilize a DC supply will also actuate 50% slower than equivalent AC valves (as mentioned here on page 512).
Solenoid valves are available in a number of different supply voltages, which provides options for different systems, i.e. a given system may not have 24 V available, so a 12 V valve would need to be used, etc. Also, as mentioned above, AC valves operate faster than DC ones.
It may also be worth considering the number of actuations requried per second and the duty cycle requried for a specific coil (some will overheat if energized continuously). Please correct me if I'm wrong on any of the information I have provided, but that is what I found regarding the subject.