Your sense of space and how to use it will be slightly different depending on the focus of your craft. If you are building furniture with hand tools, you will be able to make do with less space (under 300 sq ft.) than if you’re using mainly power tools and building sets of cabinets (over 400 sq ft.). Some 3D modelling with a program like Sketch-Up or some 2D layouts on graph paper with scaled cutouts can allow you to experiment with arranging your space and how much of it you will need. You should include not only the tools, benches and storage that you currently possess but also enough room to tackle new types of projects and new tools that you anticipate needing down the road. When you think you have a layout that works, test it out by mentally working through a range of different projects you might tackle and the needs you will have for material storage, assembly and finishing. Then revise as necessary; this process will allow you to hone in on a general square footage and layout ideas that will set you up to move forward.
Let’s be clear about one thing: No workshop is perfect for everyone. If there were, I could simply give you a precise plan to follow with clear dimensions. Every person who comes to the hobby, avocation, or profession of woodworking has his own particular collection of tools and his own peculiar work style, skills, and desires for the kind of woodworking he wants to do that gives him a unique approach as to how he will lay out a shop: where the tools should go, where lumber and supplies are stored, and how much bench area he needs.