I SAY, MAKE THAT ROOM AS LARGE AS YOUR MONEY WILL PERMIT. You'll grow into it at some point. Plus, if you have a nice large room, you can have your friends over for sewing days. Which is always fun.
My "sewing room" is 10' x 10', with a closet the length of one wall. It holds a huge sewing desk, bins of fabric, several small rolling carts, ironing board and a smaller piano.(this needs to go) I have my serger and currently one sewing machine in that room. I use my living/dining area as a studio, and have a cutting area in the garage and some additional fabric bins out there. I can get away with the living/dining studio setup as my kids are grown and I'm divorced. I can get up to 8 gals in my living room sewing as I have a conference table I got from a LQS when it closed. I have 5 sewing machines total, and have the gals use them when they come to sew QOV. It would be even more fun IF I was willing to eliminate the sofa. LOL Actually have thought about selling it and downsizing to a mobile home size sofa. No one ever sits on it anyway. I have this place for my enjoyment. and enjoy it I will.