How big is the planned house? Double the size. That ought to give you a good sized sewing room!
Seriously, it depends on how much 'stuff' you have accumulated over the years, how much of it you plan to keep and how much you plan to add.
If you can swing it, ask for a 15'x15' room. (I would love a sewing room that is minimum of 20'x20', which is a little smaller than a roomy 2 car garage.) That should give you most of the room you need. However, no matter how much room you actually have, it is never enough.
Also, would the long arm be on a frame or on a table? That makes a difference. Long arm takes up a lot more room, even a 'small' one.