I am new to this. One of the mirrors cracked so I need to replace them, how do I find out what size to order. or does it matter, probably dumb question but this is how you learn, lol,I don’t see any parts descriptions or help in this manual any help would be appreciated.
Measure one of the others mirrors 
20mm mirrors in the k40 are pretty much in everyone, look for SI or MO mirrors, SI a bit “better”, MO tougher…
Just keep away from K9 mirrors, faaaaar away.
Why stay away from k9? Just curious
@John_Stevens Bad quality, made cheap, scratches easy, fog up quick and have thinner coating.
Lower reflectance as well
Many K40 arrives from the sellers with these K9 mirrors, people who changed them see a big difference with SI-mirrors 
The extra few bucks is well worth it.