Depends completely on the device you're charging and it's voltage and capacity, it also depends on the efficiency and voltage tolerance of your boost converter and the amount of light you expect to be available as well as how often you use it. I'd use as big a set of NIMh as you can possible fit (D cells would be nice) as that gives you large over capacity from an energy storage perspective so you could pretty much charge anything you wanted from it weather or not there was sunlight if the cells were fully charged. The big thing you need to qualify exactly is, the availability of sunlight, the EXACT power requirements of the device(S) you're going to charge and how frequently you expect to use it. Without reliable numbers for those three things minimum there is no way to answer your question.