There is no standard size for propane tank fittings, and there is no standard size for propane tanks. These fittings come in all different sizes and if you need to replace a fitting, it may not always be easy to tell what size you need. \
If you know the manufacturer of the propane tank, try looking u the model name and number online. You may be able to find out exactly what size fittings you need for your specific tank this way. \