Everyone has different tastes and needs.
And different available space.
If I were building from scratch, I would go at least 12x12. And I would put it in the basement.
I would pour solid concrete walls, or, if block, I would fill them and line the inside of the walls with steel, attached to floor and ceiling. I would also use a vault door (Sturdysafe, with a dial and an electronic, with a panic release inside). I would consider a hidden egress if I had extra $
I would build an elevated gunrack on at least one wall, with shelves and drawers under it. I would make the gunrack at least 10 spaces bigger than the guns currently owned. Guns easily seen, easily moved.
I would build a cabinet on one wall, away from my bench for powder, bullets (on bottom) and primers. I would make it BIG. I would also think hard about a wide wood cabinet, with doors, with shelves able to hold "mud buckets" and Paint buckets full of brass, as well so that I could line the shelves with buckets (labeled with tape) of brass, by caliber, and process. Eg. Needs trimmed, needs sized, etc.
I would use lots of thin mount ceiling lights, and I would put cabinets with doors on it above my bench, not shelves (I am OCD and like it to look good). The bench should be 2x as long as you initially think it should be and I would leave room to get my feet and knees, if sitting, under the bench, but would have room or shelves under the bench for ammo. Consider setting the bench at a height to reload while standing...think hard about this.
Leave wall space for a deer head or three, photos, targets, etc. I would tile the floor in white, so I can find the crap I drop, easily. I would NOT put a bunch of stuff on legs in the room, cause the things I drop always rolls UNDER it. Cabinets right to the floor!
I would put 2x as many outlets as I thought I needed. And I would put half of them around the bench. I would put LED lights under the cabinets above my bench, and wire in a charging station for the radio, phone, battery drills, etc. I would make a work table for cleaning brass, and another for cleaning guns, mounting scopes, etc.
Include both FHA and AC inlets and outlets to control humidity. Mount a security cam TV screen on the wall to watch the outside world, and to secure the recording device (I am old school and don't use the cloud).
If the room isn't under a poured slab, line the ceiling with steel. Paint it all white and USE nice materials, not 2x4 and crap plywood (cause it should LOOK good and WORK good).
Now, a bit of confession. I did this. But in a 6.5x16 room. I couldn't start from scratch so believe me when I say 12x12, minimum. Think mancave.
I love my room, but I will never post a photo. Sorry.
I like the ability to look at my guns, put accessories related to each in drawers under the rack, clean a gun while vibrating cases, keep my stuff sorted and since I suffer EOS (early onset stupid) I need the masking tape labels to keep track of my multiple projects. I find myself doing 5-6 K of 223, while also loading small batches of BR stuff, sorting range pick-ups for barter, and I never stop "needing" a new gun. I might also suggest a few book shelves (the internet will eventually lose all data you need) and room for a lounge chair. I don't have room for those!
Good speakers for podcasts, music etc is nice too.
Think of it as a nice room with all the built in storage, while displaying your cool stuff. I am inspired by seeing my rifles. If they are in a safe, they become neglected.
Ok, long rant over...good luck and enjoy
