You might be interested in my recent "M" series medium tele shootout:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/137220-shootou...ml#post1443290
These lenses are all somewhat smaller than the 18-55, with the 100 of course being the smallest. I'm still loving my 120, but assuming you aren't interested in wild goose chases, the 135 is probably the most sensible choice of these. I haven't ever used, or even seen, the 150, but on paper it looks like a good choice too if you specifically want more length but are unwilling to go up to the 200.
You can find pictures of any of these lenses on this site:
Pentax Telephoto Prime Lenses
But also check out the first post on my "shootout" thread.
BTW, I see you've specifically said "primes", and far be it from me to discourage anyone from going that way. But since you implied you're willing to accept an f/5.6 prime just to get it smaller, I have to put in a plug for the DA50-200, which held up quite well against all three M's in my shootout.