Im an architect and would suggest a 6mx6m garage usually is enough for most. for additional workshop space its worth adding a meter to the end of it. Of course it al depends on what you can fit and what the council may allow you under the local planning guidelines. Some may not allow you to just replace the existing, they may have requirements for materials to use etc. What are you thinking of useing, steel sheeting or brick. Check with your local council body before you start anything as they will make you pull it down if it does not comply with their rules.
just ask if you have any further questions regarding process etc.

Justin p