Having been an Interior Designer for the past 35 years, start with an inventory of all the things you want to include in the room, Machines you own or plan to own, storage requirements in linear feet (think vertically here), work/cutting table, will it be permanent or folding. Etc! Once you have the needs switch mode to the nice to haves including how you want the room to look, ie windows, doors etc. Start with a list of everything you can imagine, then see how much you can fit in without getting too large, consider zoning the room into 2 sections, one for sewing, and one for other activities and how they will work together. Will the furnishings be built in (less flexible affects resale someday) or will you use furnishings that can be moved around. Rectangular rooms such as 12 x 18 , 20, 24 work well. The more time you spend planning the more efficient and wonderful the room will be. Good Luck, sounds wonderful already!