A typical trade-gallon thermoformed pot (the thin walled pots that are sometimes colored black inside but green or brown on the outside) measures 6.25"(top) x 5.75"(bottom) x 6.25"(height). Even though they are thin, these usually last a long time.
Trade-gallon injection-molded pots (thick, usually smooth walled) vary quite a bit but would sometimes come with about the same measurements as the thermoformed pots. These are the premium type of pot and last a very long time.
A typical trade-gallon blow-mold pot (the ones that frequently have kind of scalloped shape side) measures 6.5"(top) x 5"(bottom) x 6.5"(height). These are the fairly cheap, common pots a lot of woody plants come in from nurseries. Their rims often start to break down within a year or two. Once the rim is gone, the whole pot becomes flimsy and often starts to fall apart.
Full-gallon (as opposed to trade-gallon) pots would measure a little larger.