It is the exact size that fits the , uh , 3 1/4 x 4 1/4 (Type 405) 'standard' Polaroid back for 66x, 10x films. The kind that is used for proofing on my Hassy, Mamiya, etc.
I'm actually looking at my old 2001 B&H catalog and on page 52, under "Color & Black/White Film", "Polaroid Instant Films"...there is an entry for "Fuji Instant Film", "Fuji FP100C 10 Pk Glossy"...11.99.
And, BTW, it's 689 that is the Fuji made Polaroid the "Pro Vivid". And, yes, it fits perfectly in the polaroid holder (actually, on my Hassy PolaPlus, it seems to fit "better" (it doesn't catch when I remove the empty pack))