I ask this question several months ago and here were the thoughts given (paraphrased, of course).
Needs to be ventilated, heated and cooled. I live on the Gulf Coast of Texas.
Powder requires a most stable place.
Bench should be at least 6 feet by 2 feet and sturdy.
Area should be WELL lighted.
Area should be out of the way and controllable.
There needs to be ample area for storage.
Area should be a place where tha Hen don't have no sayso whatsoever.
Here is what I have done so far--aint bought no supplies yet but that will happen.
I had a 10X12 shed what I kept junk in, well including lawn stuff.
I converted the shed, (9 feet of it anyway) into a comfortable area for this project. I'm gonna post sum pictures sumtimes.
I reframed the inside some, built a wall across the width of tha shed, wired it, insulated it, put in a window at tha end with a room a/c and lined the walls with 3/4 and 1/2 inch plywood. Built a 6 foot by 31 inch wide bench. Top is 1/2 inch and 3/4 inch plywood glued and screwed with a hardboard top.
Filled all tha gaps and covered it with a layer of rolled on sheetrock mud. Painted it and built a small L shaped computer table on the opposite wall 49" x 53" and 21" wide.
Put in three shop lights, 6 wall plugs ( the a/c and computer are on dedicated circuits).
Have room for a lot of shelves, some installed and room for more plus a shelf for tools under the bench. I had to relocate the mower to the garage. The rest of the lawn equipment fits in the front 3 feet of the shed.
All in all quiet comfy, thats where I am this AM.
Gonna keep the powder and the primers in a space in the house.
Hope this gives ya some ideas.
Blessings