My first (and current) reef was a 10... Yeah there's less margin for error but by and large it's been a successful venture. IMHO size doesn't matter (no pun intended), but what you have in there and how well you aquascape really matters a lot. Keeping appropriate corals for the environment and really paying attention to the changes taking place in the water chemistry is key. I've seen larger tanks that are just a bore to view, but I'm happy to say most everyone enjoys looking at my little nano. 
