As you undoubtedly know, Chris, it's not how big it is but what you do with it that counts!

I often use a 15 inch non-stick Scanpan (it's actually a paella pan with deep sides) and I love it. I use a plastic chefs spoon with it, though the manufacturer claims metal is also OK to use (I'd rather not test their claim!)
I otherwise generally use 12 inch cast iron woks (for when I want to get the heat up!) and I think these are an ideal size for one to two servings at a time. Cast iron is also a great material for heat uniformity and retention.
I also use cheap aluminium pans (like the BIRs generally use) but they are exactly that, cheap (though a good conductor of heat). I'm not keen on the fact that they are soft and who knows how much aluminium is coming off them into your food!
Steel is not a good material for cooking pans, as far as I'm concerned. It looks nice but is a poor conductor of heat and scratches easily. I have one that mostly stays in the cupboard. Steel sandwiched with other materials (as these ones are) is a fair compromise though (though scratching and tarnishing remains a problem).
I'm not sure that the pan can be too big (aside from the fact that pyrotechnics - i.e. flames licking up the side - might be a bit limited, or nonexistent) but I think a pan that is too small can definitely be a problem with the food boiling rather than frying if the pan is overloaded.
Anyway, to answer your question, I think 11 inches is a good size (provided you're happy with the steel). The pans themselves appear to have nice deep sides and a robust handles.
18 quid for an 11 inch one or 27 quid for a 9.5 inch one? No brainer Chris!