The starting point is the device datasheet. That will give or point to the footprint dimensions. Bear in mind that a device may be available in several different footprints. For any Footprint in a library, you must open them in the Footprint Editor and check the dimensions against the datasheet. These documents will help: [https://docs.easyeda.com/en/PCBLib/PCBLib-Naming-Rule/index.html](https://docs.easyeda.com/en/PCBLib/PCBLib-Naming-Rule/index.html)<br> <br> and: (2.3) in (2) in: [https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a](https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a)<br> <br> See also (2.2), (4) and (6) in the same.