Unfortunately, there are no real standard sizes in the industry. Plugs & compatible jacks can vary by barrel diameter, pin diameter and barrel length. Even the polarity is non-standard. Some appliances actually use the center pin for negative/ground and the outer barrel for positive. You can determine which one is used by the pictorial representation found on the side of a gadget. It will show the pin and outer barrel and the polarity of each.
Even though there are two diameters that determine the size of the jack/plug, usually only the pin diameter is specified. For example, the popular 2.1mm jack has a barrel diameter of 5.5mm. The barrel diameter is assumed in most cases.
Despite the total lack of standards for household stuff, for the hobbyist, some popular sizes have emerged.
For low power connections, the pin diameter is 2.1mm and the barrel diameter is 5.5mm.
For higher current connections, having currents greater than 2.0 amps, the pin diameter is usually 2.5mm with a barrel diameter of 5.5mm.
Barrel length on the plugs seems to have settled at about 9.5 to 12mm long. The length match is not as critical as long as the plug is sufficiently long to make good contact in the jack. The jacks, plugs and power adapters we carry are all compatible with each other regarding the length.
In your own projects, polarity is a matter of individual choice. The convention should be positive for the pin (TIP), and negative for the outer barrel (RING). This practice is nearly universal except for some odd-ball gadgets.
Connecting the Barrel Jack
Barrel jacks have three pins – positive (TIP), ground (RING) and connection detect (RING_SW). The pin at the end of the barrel is positive, and the one coming out the bottom is ground. The third pin comes out the side and is used to detect the insertion of the plug. Referring to the schematic symbol:
The RING_SW pin detects if a plug is inserted. When a plug is inserted this pin will no longer be connected to the RING pin (Gnd).