There isn't really a theoretical minimum size, but with the best chemical propulsion possible you're going to have an initial to final mass ratio of about 5. Now that final mass needs to include the structure of the vehicle, so you can cheat a little bit with staging but you can't make it go to zero.

In practice, the smallest orbital launch vehicle ever launched required 2600 kg to launch a 4 kg payload. Turns out that the smaller a rocket is, the less mass efficient it gets.