I typically don't like retractables for most things, but I think they are GREAT when it comes to hiking, and big fields and open spaces in public places to give them more freedom... I really like the ones they sell at Walmart. They have always worked really well for us, and my boy is also bigger at 15lbs. And the handle is not huge and heavy like lots of them are. They're also some of the cheapest I've found around that are of good quality.
Walmart.com: Flexi Lead 16' Retractable Leash for Dogs up to 44 lbs.: Dogs
Flexi is the best brand, IMO.
There at 26 foot ones out there too for more money. I currently have a 16ft one for Jackson but am looking for a 26ft one. I have a 50ft rope lead for when we go to the beaches, etc, where there's not many people so he has a ton of freedom. But I love the retractable for places where there is more people so I can easily bring him back in, and not disturb other people, but still give him some more freedom. At stores, and regular walks, etc, we use a regular 6 foot nylon leash.
Edit, just found this one:
http://store.yahoo.com/pet-guys/-047181-cflong1.html
I'm gonna purchase it because it's the cheapest I've found for a 26 foot one.