What is the lowest divergence laser available, or parts to build it using a 25 mm collimation lens, irrespective of output power?
Being divergence is directly related to the diameter of the output beam, I need to specify the question to ask for the lowest divergence using a 25 mm diameter collimation lens for lasers from 400 to 1064 nm.
As far as laser diodes, it appears emitters with a size of 1 um x 5 um are about as low divergence I can find.
If 200mW are good enough: get a single mode 405nm diode. The beam parameter product should be way better than any MM diode and the short wavelength will make it even better at being collimated.
However, thats not very bright.
Get a 1,5W 520nm laser diode with square FAC from lasertree.
That should be the best you can manage in terms of divergence for a reasonable amount of power and a very high brightness.
I happen to have both of those laser diodes, the FAC corrected 525 nm 1.5 watt from Lasertree is very nice, I have a YT video showing one I used in a pointer 
I was hoping someone might be able to tell me if there is a laser diode available with fractionally lower divergence than most diodes, so far, I’m not aware of anything with an emitter smaller than about 1 x 5 μm, but when that small they are always single mode diodes at reduced output power capability.
Do you happen to know the emitter size of the lower power single mode 405 nm laser diodes? I keep looking, but not finding the info yet, seems that data is unpublished for all of them. Unfortunately, the shorter the wavelength the smaller the emitter must be to remain single mode. In this situation, the smaller the emitter diameter the lower the power, thus come the multimode diodes with their wide emitter stripes.
From what I have been able to gather or understand, whether single mode or not, IR diodes have higher divergence due to the relative diameter or size of the collimator optic in relation to the wavelength, correct?
I imagine both the emitter size as well as the relative diameter of the collimation lens to the wavelength both play a part together resulting in the divergence of a laser, but what I am unsure of is whether the emitter size/wave guide dimension determines divergence more so than wavelength being produced, although for single mode, or even multimode I expect they are interrelated.