The actual frequency can be worked out in metres, generally the antenna will be the length of a fraction of the full wave length, generally for best efficency. As far as my studdies have gone, the Rx antennas are even smaller then the usual lenghts i am use to, but that doesn't mean they won't work. it really depends on what the manufacturer determines the best compromise for the application. The thickness of the antenna will also have a bearing on the actual length as well.
The set length of the antenna is then tuned through the circuitry in the Rx for the best possiable performance for the size.
Generally shortening the antenna will shift the tunning up the band (eg. 27Mhz shortened wil move up to 28Mhz)
Thats my $0.20 worth from what I remember of antenna tunning.
It is so hard to fix the driver, but so easy to blame the car.