You've asked a loaded question. As you probably know, your dinghy is everything from the family car while anchored to a fishing boat to take the kids out in. Choosing a dink on what it will be used for is sometimes harder than choosing the boat you put it on.
I've gone through a number of dinks looking for just the right one and I've turned up with two.. An inflatable, Avon hard bottom, that folds down into a flat pack, and a fatty knees that sets over the deflated inflatable, between the mast and dogger.
Your question, "smallest", If you could get both feet in a bucket, thats the smallest, but who wants to paddle around in a bucket. I would think of what you are going to use it for first.. If its used as sail or just paddle, and do you use it yourself or will you have others with you, open water or lake use, using it every day or just now and then..
When putting it on the fore deck, you've just covered your anchor locker or your ability to get to the locker. (thats the reason I sold the first dink).
The smallest I've ever seen was about 2 feet wide and about 3 feet long, and used by a guy getting back an forth from his boat in Sasusalito,
(San Francisco Bay).. I have also sean a few "Seveyor" inflatables, at about 4 feet long..
With a smaller boat, you dont have a lot of options.......