I actually already have that trick used to log all events, so I can tell you that since I booted I've had 640 events processed.
Just ran mdevd-coldplug again, and now I have 1172 events logged, so that's 532 events generated by mdevd-coldplug just now. In my log I have basically the output of "date && env && echo ---" and based on said log, those 532 events are around 120 KB. As for the 640 original events, around 140 KB. It seems it should all be under the 192 KB of the default buffer, but as observed with that buffer mdevd runs out of space... 1 MB is propably too much, I should try to see how much is actually needed. Next time I get to do some reboots I'll see what I can find. Cheers,