I occasionally see dishes on towers pointing horizontally, which I assume are long distance microwave links. Can anyone give me an example of frequency used and beam angle in radians of the directional beam?
Frequencies can be anything from a few GHz to a few tens of GHz, and 3dB angles depend strongly on frequency and dish size, usually you don't want to go much below a few degrees in the main beam as that can get very hard to align over a long path.
Antenna gains can be north of 30dB at each end of the link, so power input levels are often less then a watt, because you don't need more for most line of sight paths at the bandwidths most of these radios operate.
For example Ubiquiti (Which is the cheap stuff) do a 75cm dish for the 5GHz band that has 31 dB of gain and a beamwidth of maybe a few degrees or so, with maybe a watt up it and a 40Mhz channel bandwidth I would expect it to be workable for any tower based path at basically constant altitude (Curvature of the earth would likely be the limit if an intervening hill wasn't).
For a given dish diameter, the bean tends to narrow as the frequency increases, so a 24GHz dish will usually be less then a foot or so in diameter just so the beam width remains something sane.