>I wonder if a radiator beams more as the wavelength approaches the size of the driver.....<
That's exactly what happens. When the diaphragm is small compared to the wavelength that's being reproduced, it's omnidirectional. As the wavelength decreases, waves from one part of the diaphragm go out of phase with waves from other parts and start to cancel, making the diaphragm increasingly directional.
That's why woofers, which are small compared to the wavelength, are omnidirectional, while tweeters tend to beam.
>I also wonder how various linear designs work into this. A ribbon tweeter is a point, but stretched up/down for a distance.<
One is called a point source, the other a line source, and it's been said that they're the only correct ways to design a loudspeaker.. The point source is omnidirectional, and the sound falls off as the square of the distance. An infinitely long line source OTOH radiates in a cylinder, and the sound falls off linearly with distance, rather than following the inverse square law. Of course, you can't have an infinitely long ribbon tweeter any more than you can have an infinitely small point source driver. But the floor and ceiling reflections of a line source actually make a line source behave much as if it were infinitely long -- imagine mirrors on the floor and ceiling, the line source would seem to stretch out above and below to infinity. If this weren't the case, line source speakers would act as omnis at low frequencies, but instead they maintain much of their vertical directivity.