Tanker size is typically measured in deadweight tonnes (cargo capacity for carrying water).

Here are tanker sizes and typical abbreviations (in deadweight tonnes)

GP General Purpose 16,500 - 24,999

MR Medium Range 25,000 - 44,999

LR1 Large/Long Range 45,000 - 79,999

LR2 Large/Long Range 80,000 - 159,999

VLCC Very Large Crude Carrier 160,000 - 319,999

ULCC Ultra Large Crude Carrier 320,000 - 549,000

Also:

Handysize 35,000

Panamax 65,000

Aframax 80,000

Suezmax (Million barrel) 130,000

Capesize: Any vessel, usually carrying dry bulk cargoes, that is too big to navigate the Panama or Suez Canals.