How big? That depends on a ton of things, like how much land you have, what you are going to be building, how many machines/tools, etc
I am currently building my last/final shop, it is a 28 X 28 building, I get half. swmbo gets the other half, here is my layout:
Attachment 300485
After 30+ years woodworking I have discovered what works for me. For instance, I have no need of a jointer. Do that on the TS. Do not need a planer, use a flat panel sander instead. Do not do a lot of sheet goods so my TS can be up against the wall without a ton of space around it. BS is rarely used, so I put it in a corner. I use a RS a LOT, and I mean a LOT! So it is right near my bench. This shop would not work for a lot of people because of how they work and what they build.
Someone said build your shop twice as big as you think you need. I do not agree. If you spend a lot of time in a shop, not just puttering around, those big shops wear you out with all the walking around. Nevermind having to move stock continually. I would advise that you build in such a manner than you can expand easily if need be. For instance, you can pour a much larger slab than you need, put a shed roof over it and enjoy the shade. When you need to expand, wall it in, concrete is already done.
I would advise anyone to create overhead storage space. Mine has a set of stairs in the wife's shop going to an attic with 7' ceiling. Handy to store all sorts of rarely used things.
So, tell us what you want to be making and maybe we can all give you some tips?
Edit: I have built lots of nice furniture in my last shop that was layed out like this. Complete kitchen, three bedroom sets (plans from WoodSmith), lots of high end boxes, shelves, you name it.