My neighbour has a fabulous sewing shed which was originally a playroom for her grandchildren. Insulation and heat are essential. Condensation is the killer with any outside space.
One thing she has done which is an excellent use of space is use a long (4m probably) , narrow length of kitchen worktop along one complete side. It is supported on three 3 drawer units and is heavy enough to sit on top of them without shifting. Her machine, overlocker and coverstitch are all out permanently with 9 drawers that she can access without even getting off her wheeled chair.