Like all things there is a limit to how large you should be thinking of going if you are using the DIY route with your projected new conservatory. For example a 4000 by 4000mm Edwardian would be fine. In fact you could go up to 5 metres on the width. The same sort of thing applies to the other styles such a a lean to or Victorian.
What you should be aware of is that going much over this kind of size makes the construction less of a safe DIY project and more of a job for a professional window fitter. This is particularly the case if a glass roof is involved. To do this kind of job safely you will need more than just a pair of step ladders!
Another thing to considder is that beyond a certain size, the roof framing changes from a standard roof to a portal roof. Again, a portal roof would have to be professionally installed and also cost thousands of pounds.
Using these guidelines and staying within the recommended size limits, your self build will be easy to put up and the costs will be kept to a minimum which is after all what a self build is all about.