Evaluate Spacing
The biggest consideration when shopping for monitor size is the workspace itself. With a measuring tape, determine the size of the space where the the monitors will be placed and write it down. Also measure the distance between the spaces, as the monitors should be set equidistant from one another when listening to mixes. If monitor stands or wall mounts have been purchased, that will automatically limit what kind of monitors will be bought depending on the base size of the stand.
Consider Portability
Think to the future. Is this the way the studio will always be situated? Is it likely to move around? How easy it is to move the monitors once they've been placed is another consideration if you happen to live in a multi-unit dwelling.
Calculate Wall Strength and Thickness
Wall strength becomes a factor when you want to use wall mounts to set the monitors. If you have drywall, wood paneling, or something similar, the weight and size of the monitor become extremely important. Too much weight may not show itself immediately, but over time, it will wear and tear on the wall through sound vibrations.
Measuring wall thickness will not only determine how much weight the wall can support, but how the sound will be diffused, dampened, reflected, or dampened which can affect mixing.
Include Your Neighbors
If you live in an apartment complex, duplex, or any residences that are attached to one another, it will limit (or should) the volume to which you can turn up the monitors. Larger monitors are, by and large, able to output more sound than smaller ones and even at relatively low levels, the bass will travel. Trust me--the neighbors will be the first to sound the alarm if it's too loud and could care less if the muse is hitting right at that moment.
Ask A Pro
Some music shops have very good customer service. Giving detailed information concerning what has been discussed will give them an idea of what monitor would fit for your situation. One of the best is Sweetwater (www.sweetwater.com).