Size is definitely hard to estimate without knowing what you want to have however, if we talk about motion platforms, they come in all shapes in sizes, especially the DIY ones.
Here is one of the “Cheap DIY motion seat mover” concepts that is well shown. He has a thread on the forums about it and various youtube videos showing it’s capability, price, and he also has cad files for his projects:
This setup is definitely one of the more popular options due to the cost, ease of construction. Because of weight restrictions (stall motor torque) Equipment is often limited to a chair, basic flight controls and VR (you could probably stretch in a monitor also on your seat mover but you will start hitting stall torque of motors, you can also keep the monitors outside of your motion seat however the immersion is more easily lost.) but as said before, is insanely cost effective, it’s also very size competitive, it’s not much larger than a general office chair with leg room to spare for rudder pedals. I would give a 120kg estimate for the amount a seat mover like that would be able to easily move, which if you’re light you could probably make it work but it would be a real challenge.
Now, if you want a better motion platform to put a replica sim rig on, non-commercial options like these are always good to look at for a reference to scale, as they provide many photos and images of the build process for DOF 3,4,5, and 6.
DIY Motion Simulator Projects (xsimulator.net)
There are all types of motion platform types there, you can also modify them within reason, you could make a forum post there with your specific needs and someone who definitely knows what they’re talking about would be able to critique/improve your design.
It also will heavily depend on the type of plane you make; I made a C172 simpit, the width dimensions for the panel were 1.05m, and with a chair I was looking at another 1m for length. The panel was about also another 1m from the floor. And a C172 is classed as a “small plane.” So ideally for a large range of motion you would need a pretty big range for linear actuators or motors which means they have to be further away from the platform to mount everything to. Fortunately, if your plans were VR, the saying often goes less is more, in terms of motion range there, it’s easier to fool the brain.
I guess if all else doesn’t meet your requirements for a baseline size, looking at a commercial option’s overall size would also be able to give you a decent scale.
Also make sure in the cost of your shed build that you have a way to route power and internet to your shed (often a wire from the house underground as my shed had done.)