My boys have a (puppy) medium sized kong. It's a tad bit too small for Kai so I'd get at least a large for Jasper.

For recipes, you can find a few here.

Kai actually never really showed interest in his kong until tonight and we've had it since he was a very young pup. I just needed to find the right food to stuff in there. I tried putting in some freeze dried liver bits in there with some peanut butter to hold it. WOW! He went NUTS for it. He never really liked peanut butter but he absolutely loves liver so I guess you'll just need to find the right recipe.

[edit] Sorry, forgot to answer your other question. With a larger sized kong, it wouldn't be too difficult to clean. I just find it a bit fustrating to clean peanut butter out of a medium sized kong but you can always avoid the PB and put some less messy food in there.[/edit]