using the gungans' shields as an example, the shields stop anything entering them that are moving faster then X speed. these are kinetic shields which is why the droids must walk into the shields before attacking. a ship leaving the atmosphere must reach a certain speed to break through and into space, which is why if their shield was similar to the gungans aka a kinetic shield that speed needed by the transport ships was to fast. to a kinetic shield size maters not, as long as its moving under the barrier threshold. From another post blasterspeed speculates that a blaster shot is moving at 150meters per second. that translates into around 350 miles per hour. so the kinetic barrier is at least stopping things moving that fast, however i would hazard to guess it is stopping things moving closer to 75 miles per hour, as thats one of the lowest speeds an airplane can fly out, hence preventing air vehicles from entering the shield. ( thats assuming a tie fighter can fly that slow i guess they could slowly fall into the shield as long as they were under the speed threshold however it would be difficult, plus all the rebel guns shooting you while you came to a halt. )
edit, after relooking at the blaster speed question, another person says they actualy are moving at 35 meters/second which is about 75 mile per hour, agreeing with my minimum flight speed.