Trim it down, and don't worry too much about it. The film is orthochromatic so it can be handled under a safelight.
Just to be sure I use the dimmest safelight I got.
Also, the film is around ISO 8 (I think the paperwork says from 6 to 12 depending on the light used), so don't worry about pulling one out of the bag, trimming ans putting back in the bag.
BTW, a 4x5 sheet film holder is really 3 15/16 x 4 15/16 inch.
1/16th of an inch is about 1mm.