I know this is a really old post, but I had to try to help out. :)
I've been looking into squab production myself, as of late, and from what I've found, the utility breeds are ready to butcher in about 5 weeks, or 22-ish days, depending on the grow rate of your specific breed. Most wait until after the babies leave the nest, but others will butcher right before that. I personally will probably wait until after they start trying to fly, because I like the flavor of meat that has been used for it's intended purpose (exercised) before it hits my plate.
As far as weight is concerned, that also depends on the breed of bird, due to the variance in the weights of the different utility birds. I'd guess that a good baseline to go from would be that the intended dinner be at least 3/4 the weight of the sexually mature birds of the same breed.
I don't have pigeons yet, but I do a LOT of research before I add any additional stock to my citystead farmlette.
iI's not just pigeons that people do this with. I bought some Silkies from a gal who was positively mortified that I was planning on eating any of my Silkies, even if it were to be only the ones that the chicks she sold me, produced after they reached adulthood. She actually told me that "Silkies aren't for eating"! 0.o considering that they are a delicacy in the east, and have been proven to have certain health benefits over white meated-chicken, I'd say that they are definitely for eating! LOL
And for personal commentary: First post! I've been perusing this site for months now, and just now felt the need to make an account. :P Count me as finally out from under the bridge. LOL