You're designing a building on an Architecture course? From that I can tell you aren't at Greenwich or East London, where you can do 7 years without designing any buildings at all. There's a reason I know that.
To answer your question, I need to ask one of my own. Is this for a hobbyist in his garden, or a commercial enterprise? A hobbyist/ one man band operation would be quite happy with 2.4m clear, meaning a sheet of ply or MDF can be flipped over end-for-end. A commercial operation would probably want at least another metre of height, and maybe more. If you are building in storage (woodrack etc) then you might want to think about a mezzanine area which is going to take the height to 5 metres or more. Basically, sort out the operations that are happening inside and (within site limitations) the floor-space and volumes will follow. Plan depth is also an issue, if you want to get natural light into the middle of the building, and any fancy roof shapes that result from trying to bring light into the middle of a large space will also add to the height.
My over-arching suggestion is to try to constrain yourself. It's easy to say "here's a whopping great building which will serve all the necessary functions", but much more interesting to ask" what's the smallest building which will work?".
