I just looked back at our K1 visa application (1-129F petition): 83 pages. In addition to the required stuff (passport pages, letters of intent to marry, etc.) I also sent documentation of all of our visits to each other (receipts, boarding passes, photos of me with him and his relatives, bank statements showing charges from trips), and a section of evidence of ongoing relationship (phone call records, emails, handwritten letters, a few affidavits attesting to our relationship from people who knew us). Not a ton of each thing, but a well-organized (I used section cover pages listing everything in each section, and also labeled everything with locations, dates, people shown, etc.) representative sample.
(Just realized this thread is in a ROC forum: ROC package was 107 pages; list below.)
No RFE; no issues at Nairobi (which can be a tough embassy), and never interviewed again until naturalization 8 years later.