The gas tank size for a 1973 Dodge W100 stepside pickup varied depending on the specific model and options. There wasn't a single standard size. To know for sure, you'd need to check the owner's manual for that particular truck, or look at the truck itself (if possible, the sticker on the gas tank door sometimes indicates capacity). You could also consult a Dodge parts catalog from 1973.
However, a common size for that era of truck was around 18 to 20 gallons. Keep in mind this is just an estimate; it could be slightly smaller or larger.
