Without weapons or anything, you can get a fast fighter at two hullsizes, that'll likely do to conceal it for a while, until the planet has strong Anti-Missile sensors on all the time.
But if they are hostile already, what do you care?
You can't change what they have on that planet by virtue of espionage, if you kill their fleet you can normally survey the planet without problem, and population size and economy can be roughly extrapolated by means of the emissions.