I got a table recently, was debating between 8' and 9' but since I'm a serious player I really wanted a 9' or nothing.
Minimum size is playing surface + cue length (x2) + minimal backswing length (x2) in both directions.
So for width, 50 + 58 + 58 + 2 + 2 = 14.16 feet
Length = 100 + 58 + 58 + 2 + 2 = 18.3 feet
That would be with a very short 2" backswing, but that would only come into play when shooting on the rail and directly across the table. Typically you are not on the rail, and/or not shooting directly across or up the table.
My room was 15'6" wide but only 17'8" long. So I'm about 4" short in both directions lenghtwise, and I can tell you it rarely comes up. I have an open railing on one side so can elevate if necessary, or use a 52" cue as needed. And for drills, normal ghost-type practice, etc., it's perfectly fine. So happy I went with the 9', practicing on an 8 and playing on a 9 would have been a horrible idea, for me at least.
Sure it would be nice to have more backswing space, and more room for furniture etc., and next house I will look for that. But since we moved into this house when I wasn't playing pool (took 7 years off), that sort of space wasn't a consideration. Plus nice rooms for my 5 kids were slightly more important... In the meantime I have a great, professional table to practice on with very little accommodations.
So take a realistic look at the minimum space you need, not the "standard" recommendations, and decide what's important to you before making the decision.
Scott