@annieeg Sorry, I briefly saw this yesterday, then completely forgot to answer!
My Brother 881 is 42" wide, and then needs about another 9" either end for the carriage to get to the end of the needles. They need a fairly sturdy base, and they clamp to the edge of the table, so you wouldn't want them on your best dining table.
I have one on a knitting machine cabinet (known as The Coffin, it's huge!) and I also have a knitting machine table that has folding legs for easy storage. Ideally, you need to be able to set the machine up somewhere and leave it, they are a bit unwieldy to keep setting up and taking down, especially if you have a ribber attached too.