Hello Delancy. The answer to your question depends on where you live (metropolitan vs. rural area). If you live in a gated community or other well defined subdivision, the CMA should use data from only that subdivision. I usually start with a 1/4-mile radius and work my way out to a 1-mile radius if necessary (in urban areas). In the suburbs, I'll expand beyond the 1-mile radius and in rural areas, where properties are farther apart, I may expand as far as 20 miles.
To determine what's comparable, I'll look at square footage, bedroom count, lot size, age and style (e.g., single story vs. two-story). I try to find homes that have similar features as the subject property (e.g., swimming pool, garage size). Once I have selected the homes that I want to use for the CMA (ideally 3 active listings, 3 pending listings and 3 sold listings), I'll take a look at all the features and I make the necessary adjustments to indicate whether a comparable property is superior or inferior to the subject and then I choose the one that are most comparable to the subject. As you can see, preparing a CMA is time consuming if it's done right and having access to data that provides you with the necessary detail is crucial. I hope this information helps.