I have that lathe - it can't do an exact foot in diameter
It's a lovely lathe mind, I don't have any complaints about it, but 11.5" is the maximum diameter I've managed and it was pretty evident that I was pushing it at that even though the blank was only 2" thick or so. Squeaky bum time.
Mind you, how much of the squeaking is down to that being a hot melt glue block holding the platter there at the end while I took off the tenon I'm not sure. Glue blocks. Crazy how much those things will take.
Mind you, I've got a smaller chuck than MB's, I'm using what rutlands now sell as the Precision 75 (it used to be the Xact 3) so a 3" diameter chuck. The lathe will handle a 100mm chuck, maybe even the 114s that axminster do, but they're spendy (I got two of the rutlands ones for 80 quid by buying ex-demo stock). If you're doing large stuff; go spendy. If you cheap out like I did (well, technically I sank money into having a few different kinds of jaws instead of one set with huge capacity), don't try to find the lathe's limits.