Firstly what do you require the conservatory for? Perhaps a dining room where you may wish to consider a wide conservatory of an approximate size say 4 m x 3.8 m so you can place a table across the conservatory and enjoy the garden views when entertaining, or something smaller say 3 m x 3.5 m with some comfortable conservatory cane furniture to relax to read the Sunday papers and having an afternoon nap.
Marking out the outside of your conservatory is always a very good idea so you can see how big it will be. Set out this area with garden canes or string lines, then put some garden furniture in this area to represent your proposed furniture layout, this should give you some idea of how big the conservatory will need to be and you can see how much garden it will take up. This will help you decide on how big your conservatory will need to be. Also it will help you deciding on your patio and landscaping arrangements.