I doubt I would ever use a sewing room that wasn't attached to my house. I have read lots of people have their rooms unattached. I hope it will be all you want it to be.

Having recently repurposed an outbuilding into a sewing room I would second what someone already suggested about having the outlets placed higher on the walls. So nice to not have to crawl under the table to access the outlet!

I agree as large as you can afford, I love mine!!!!

Congratulations! A newly built craft studio to be. Give your DH a hug from me.

I would have lots of natural light, maybe a sliding door to step into your garden when the weather is nice, and closets with lots of shelves to keep my fabric, etc. out of the light. I just turned a bedroom into a quilting studio, and would love to have more closet storage, I have lots of natural light. I enjoy my room totally. if you have room, include a comfy recliner with a good reading light, and maybe a tv or built in speakers if you listen to music.

Recessed outlets are awesome so location of outlets does not limit furniture positioning (you could push a bookcase against outlet and still plug things in)

All great ideas. The main thing is to go as big as you can, 'cause stuff allways expands to meet the exsisting space:D

Don't forget the heating and air conditioning.

There is a terrific book by Lois Hallock "Creating Your Perfect Quilting Space" that might be helpful. Of course looking at all the wonderful examples here on the board would be helpful, too. My room is 12x14 and I would love it to be at least twice as big.
Lots of natural light. PLENTY of wall space (you can never have too much design wall space!). As many electrical outlets as you can get and do not let them put them down at the baseboards - who wants to be crawling around on the floor all the time to plug stuff in. If you have a modern sewing machine (lots of computer boards) be sure to have a dedicated circuit for that so you don't run the risk of blowing a fuse or having a brown out on your machine.
Other things --- ceiling fans for summer --- radiant heat for those cold winter floors.
Yummm! Have fun and enjoy. :D

I used Lois Hallock's book mentioned in the post above as a guide when DH and his friend finished off a part of the basement over six years ago to create a wonderful sewing space for me. It is 15 by 24 feet at one end. The other end is like a TV/stereo area along (so that DH could keep me company!) with a two-piece bathroom. The book was an invaluable aid in making use of what you already have before buying new things, and very good ergonomic information was given so that I could iron, cut and quilt at the frame without any discomfort. I love, love my studio!

Electric outlets low on the wall or built into the floor. Big enough for a long arm lighting, lighting, lighting, cooling and heat, a cutting station, an ironing station and more than one sewing station so you can invite friends to play. Then there is always lighting, don't forget that. lol