In addition to sheer power, look for a model with a built-in thermostat so you can set it and forget it. This convenience feature is well worth it so you don’t have to stop what you’re doing mid-project to manually turn your heater on and off to maintain your desired temp. A good garage heater will mount to the wall or ceiling to save space and will come with a durable housing and full safety screen to keep dust and wood chips from reaching the heating elements and starting a fire.
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.