In his interview with Bob Porter, Lt. Day mentions a "3 by 5" card", probably a standard size card for most prints. The palm print was lifted with 2" wide tape, so the paper it was mounted on was non-standard.
This illustrates nicely that "Day said so" (especially 33 years later) doesn't make something correct.
Summary from Pat Speer of how Day's story evolved through the years:
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1. 11-29-63--4-22-64 Lt. Day's initial story regarding the rifle is that he removed the wood stock after noticing a print going under the wood stock down at the bottom of the barrel near the trigger guard, and that he then discovered a print that had been completely covered by the wood stock near the firing end of the barrel. He says he then lifted this print, and was pressured into turning the rifle over to the FBI before he could photograph both what remained of this print and the other print on the barrel he hadn't even started to work on.
2. 9-9-64 Lt. Day stops claiming he removed the wood stock from the rifle after noticing a print on the barrel by the trigger guard. As this print was not lifted by Day nor developed by the FBI, the FBI's failure to observe or document this print was quite a problem, and its disappearance from Day's story within days of the FBI's telling the Warren Commission they found evidence supporting Day's claim CE 637 was lifted from the rifle... is quite the coincidence.
3. 9-9-64 Lt. Day names the person he claims pressured him into discontinuing work on the rifle, and it's Chief of Police Jesse Curry.
4. 10-18-77 Lt. Day begins claiming it was the print down by the end of the barrel--the print he claimed he lifted--that he observed before removing the wood stock. This is quite the change considering he originally claimed this print was completely covered by the wood stock.
5. 10-18-77 Lt. Day also begins claiming it was Capt. Will Fritz, as opposed to Chief Jesse Curry, who pressured him into discontinuing work on the rifle.
6. 10-18-77 Lt. Day also begins claiming he told FBI agent Vincent Drain about the print on the underside of the rifle barrel before handing him the rifle on 11-22-63.
7. 1993 Lt. Day begins claiming it was Capt. George Doughty who pressured him into discontinuing work on the rifle, as opposed to Curry and Fritz.
8. 1993 Lt. Day also begins claiming he not only told Vincent Drain about the print on the underside of the barrel, but pointed out its location.