There are many ways to classify the sizes of launch vehicles.
The Augustine Commission that was created to review plans for replacing the Space Shuttle, used the following classification scheme:
A sounding rocket cannot reach orbit and is only capable of sub-orbital spaceflight
A small lift launch vehicle is capable of lofting up to 2,000 kg (4,400 lb) of payload into low earth orbit (LEO)
A medium lift launch vehicle is capable of lofting between 2,000 to 20,000 kg (4,400 to 44,000 lb) of payload into LEO
A heavy lift launch vehicle is capable of lofting between 20,000 to 50,000 kg (44,000 to 110,000 lb) of payload into LEO
A super-heavy lift vehicle is capable of lofting more than 50,000 kg of payload into LEO
