As folks have said, whatever you build, it will be too small sometime in the future. I've recently built my workshop, 7m x 11m was all I could manage. This is my 3rd shop and it's benefitted from what I've learned with the rest. Here's some tips in no particular order. If you have to prioritise, do things now that cannot be easily changed later. Insulate the floor. I didn't do this once but laid a slab with foundations 1m deep at the edges. That's a lot of concrete and it ran with condensation in the spring months. Don't forget to insulate the slab edges. This does leave a strip of insulation exposed all around the shop. I screwed 2mm galvanised steel strips over this then painted it with the floor. Height is useful, especially the door if (when) you want to get vehicles in there. 4x4s and caravans are quite tall things. Insulate and panel the walls. I used 50mm glass fibre between battens then 18mm plywood over that. Paint everything inside white or light colours. It makes a big difference. I used cheap emulsion on the walls but over painted with gloss where oil splashes were expected like the lathe etc. Fit lots of lights but switch them in groups to save electricity. It also means you can run a bigger machine if only a few lights are on. Add extra lights where detail is important like bench, lathe, drill, etc. Don't scrimp on security. Everyone knows a nice new workshop will have nice new tools in it. Secure doors, good locks and an alarm should be the minimum. Fit BIG sounders inside the shop. If the thieves=92 ears are hurting, they will leave sooner. You can make your own security doors by bolting a sheet of ply either side of a 3mm steel sheet. Don't forget to rebate for any locks when you make it though. Making a rebate after it's all glued up is a real ****. It helps if the outside of the shop looks a bit scruffy. Also, any windows should have obscured glass. I only fitted translucent roof panels, no windows. That means more wall space for shelves etc but still lets in the light. Get a good floated floor finish then paint it. I used a light colour that didn=92t match anything I'm likely to spill (oil, blood, etc). If you power float concrete, it's the best you can get but you will need to etch the surface before paint will stick. Run all the wiring on the surface of any wall covering. That way you won't hammer nails through it when you hang things up. It also makes it easy to add extra sockets etc. Think about the internal layout before you build. Putting things in corners can limit their use if there's not a convenient door or opening. Think of a long piece in a lathe with the headstock against a wall. The same goes for power saws, drills, mills, shears and hydraulic presses. Heat is a big bonus but a large shop with a thick concrete floor takes hours to warm up. Insulate the roof and paint it white if at all possible. There should be a ramp up into the shop to keep rain out. Any lights should have guards to protect against you wielding lengths of material and any debris from the lathe/mill/drill chuck Hope this helps.
John