I had prescription lenses installed in folding readers, so my case is about the size of a cellphone. I haven't weighed it, but it's definitely less than a normal case. Here's the link of the place I bought them from: http://storefront.myspex.com/

They're not cheap, something like $75 when I bought them. Still, they're the cheapest folding frames I found that advertise as "optical quality", which is important for prescriptions.