My work shop is about 20' x 20' and I've been working in it for decades. I use the overhead space for wood storage and have shelving and storage cabinets down one wall. I have a table saw, lathe, cabinet makers workbench, band saw & drill press. A couple of walls full of hand tools. The space has worked well for me except when I build a large bed or table, per LittleBen's comment.
If you're going to use the space for lawn mowers, garden tools, etc, I would make it 24' x 30' if you have the space and funds. No matter how large you build it, you'll fill it up. Just plan the best you can and use the space efficiently.......and insulate it well.......and enjoy!