Chekov caught a glimpse of a Bird of Prey in "The Search for Spock" and described it as "a scout class wessel."
That should give you a range of sizes, at least by crew: a scout will have more than one and less than twenty, probably about runabout sized or larger.
To which Kirk replied: "Could be Grissom
There's debate about regarding the size of the Grissom
but it would seem that it, too, qualifies as a scout.