The footprint of a longarm/midarm is bigger than you would think, so start there. I had a 15' x 20' room which was perfect, until I got the longarm. Then I had to get really creative - my cutting station and big board ironing board were on wheels, so I could move them out of the way when I needed to longarm.
If I could build a new sewing room, it would be a minimum of 20' x 30'.