There are some wierd and wonderful shaped awnings, but if you are looking for the 'traditional' shape, then 3m is generally the biggest depth. There are a variety of awnings that come in this size: the Apache Buckingham; Pyramid Orion; Ventura Pacific 300; Inaca Mirage 300; Isabella Commodore Alpha Royal; Isabella Opus 300;
There are the odd one or two larger at 3.5m: Dorema President Vario 350 with the option of turning the front bit into a sort of porch; the Quest Palace (which looks a bit similar to the President in so far as you can indent one bit of it); Isabella Penta; Isabella Penta Concept; Isabella Forum;
Isabella have brought out a 4m awning, but as far as I can tell 1m of it may be a canopy - it's called the Isabella Vision Awning 2010 and they have them (or can get them) at Derby Caravans.
When purchasing a large awning, always remember to consider you would need to find a pitch which able to take it.
You can get isabella ones that are 350 deep, as well as the 3m orion ones.
Orion 3m doesnt appear to have a very good rep , I read they sag at the roof, not enough support and poor quality, but they are cheap .
The isabellas are very expensive and excellent quality, so I guess you get what you pay for.
I have seen some on seasonal pitches that have the option of using as normal depth awning or adding the extra depth on as well.
I wouldn't fancy putting one of these huge ones up for weekends , great for long stays or a seasonal pitch though.