Liners can vary greatly in size. QandZ and Shady Oaks are the best sizes. But it can also depend on the popularity of the variety. Sometimes we get liners that are ready to go into a gal pot and others are very small.
And it will also depend on the time of year you order. Hostas from the g reenhouses are just breaking dormancy, so they are smaller, but in May and June, they are much larger.