I have the poco telephoto cycle "c". Which, is on the 5X7 models. I have a few of the original holders. They, as per your description, are infact larger that 5x7. I have metal inserts to accept film rather than plates as the camera was designed to take.
Just to head off any potential confusion here, I should point out that I was told by someone knowledgeable that while most R.O.C cameras like mine (Universal, Premo, etc.) can use each other's film holders, the Pocos use different holders that won't fit.
The Premo 5x7 holder I have seems identical in size to "modern" (i.e., Fidelity, etc.) film holders.
Yes, you're talking about film sheaths, which are widely available for plate holders. I was also told by the same person that film holders (as opposed to plate holders w/sheaths) were widely used when these cameras were made and are still fairly abundant. Now if I can only find some in my size ...