Here is additional food for thought (see links below as well). I was in the same situation as you and here is what I found.
I would suggest that the two big questions (I am just adding to what kvcummins said) are:
- What do you want to do with your build?
- Based on your personality and shop space, how likely are you to “try later” to re-size?
Let me explain. Begin, perhaphs, by reviewing my post/repsonse here (MPCNC build in Montreal, CA - #6 by thejat).
So for me, I wanted just to carve some basic signs in pine (so super fine cute accuracy may not matter as much). Also, wanted to at least create some average sized signed. So I went with a 24" x 24" work area out of the gate. All in all my MPCNC works fine/great. Although, since I added dust collection, I actually get about 600mm x 575mm which is less that 24" x 24". Hence I really did not get my originally size anyway 
More importantly, if you look at the picture of my rig in the posting above, it has an enclosure and cable chains and is in a small space. Based on that, and my personality, the likelihood of me trying again later is very small.
My point is, I think you have to consider both of the above in deciding on size. Hope that makes sense.