Determine an oil pan drain plug's size by either test-fitting sockets onto it or using a service repair manual. If a repair manual is not present, eye the bolt and try out sockets to see if any of them fit snugly.
Drain bolts usually range in size from 14 millimeters to 19 millimeters depending on the type of vehicle. The bolts heads are usually six-sided and not hard to remove. Even on vehicles that are rusted beyond being drivable, oil pan drain bolts are still very easy to remove due to the lubrication inside the pan.
The first way to check the socket size is to test fit several sockets. Most mechanics can eye the bolt easily and determine which socket it is. Place each socket onto the bolt and wait to see if it fits without any play. Pull the socket back and write the size down to remember it.
The second way to check the size of a drain pan bolt is to purchase a service repair manual. Service repair manuals contain all the information needed to troubleshoot and repair many of the systems found on a vehicle. Manuals will also list the size of the bolt and tools required to remove it.