Depending on what your building the rivet sizes vary. This is due to the different front and rear trunnions that are available. About the only rivet that is consistant with all builds is the triggerguard and centersupport rivets.
I have provided pictures of the rivets purchased in a group buy. The first is the front trunnion swell-neck rivet (it goes into the countersunk hole). The second is the long rear trunnion swell-neck rivet. The third is the triggerguard rivet. The one picture I don't have is the non-swell-neck front trunnion rivet. For that I have been using the McMaster-Carr P/N 97300A105 which is .272" head diameter, .112" head height, .496" OAL, and .153" shaft diameter. Hope this helps.
I have the specs on the K-Var Bulgarian rivet sets at home. I thru a set of calipers at them the other day and ordered rivets based on those demensions. When I get home tonight I will post the specs to all the rivets in that kit.