As most people will say here, go as big as you can and that your equipment will allow. Even with extracts, the closer you can get to a full boil, the better. The real issue more than likely what your burner situation is. Many stoves will have a hard time getting 4 or more gallons to a boil, or at least take a very long time. That alone will limit the size boil you do.
As far as recipes are concerned, you can offset the smaller boil volume by doing a late extract addition. This will allow you to get your hop utilization pretty close to what the original recipe calls for.
To give you an idea, I use a 5.5 gallon pot and use 4.5 gallons of water + extract. That gives me about 2 inches of head room and after boil off I only have to top off with a little more than half a gallon to get me to the 5 gallon mark after the boil.
I don't know what your shopping options are across the pond, but my pot mentioned above is stainless and cost around $40 at a typical department store.