I may not really understand your question.
If you use letter-sized tickler files, it makes no difference whether you put a letter-sized document in it, or a 3 x 5, or any other size.
I have a letter-sized tickler file. I put docs of all sizes in there -- postcards I get announcing upcoming events, coupons I want to think about using before they expire, doctor appointment cards (business card size) and some 2 x 2 notes where I captured my thoughts. I have a stack of 2 x 2 notes on my desk as a capture tool.
In the beginning I put dollar bills in the tickler randomly as an incentive to get me to check the tickler every day. I also put in randomly drawings my kids made. This way, on some days I get a warm fuzzy moment when I check the tickler and pull out a drawing.
As I have refined my use of ticklers over the past 8 years of using them, I now have only 12 ticker files, one for each month, that I check on the first of each month. It just suits me better this way, I have no strong argument for or against the more traditional 43 files. If there is really an important date-related document, I put an event in my Lotus Notes calendar to remind me, with the hard-copy in the tickler. This way I see it coming in my weekly reviews.