A photograph can satisfy every rule in the specification and still be rejected for being 2 mm too small, because printing is where digital accuracy meets an application that thinks it is helping.
The setting that ruins it
Print dialogs default to fit to page, scale to fit or borderless. All three resize the image so it sits nicely on the paper, which is exactly what you do not want.
Typically they shrink by between two and five percent to leave a margin - enough to take a 34 mm head down to 32.5 mm and out of the band on a UK application.
What to look for before pressing print:
- Scale set to 100%, or an option called actual size.
- Fit to page off.
- Borderless off - borderless printing enlarges the image to overflow the paper edge.
- Paper size set to the actual paper in the tray.
- No "shrink oversized pages" option enabled.
Print a sheet, not a single photo
Do not print one 35 by 45 mm image on a sheet of paper. Print a sheet - a 4 by 6, 5 by 7, A4 or Letter page with several copies laid out on it at true size, with cut marks.
Three reasons. Most applications need two or more identical prints, and copies from one sheet are identical by construction. A full page gives the printer something to register against. And cut marks let you trim accurately, which a lone photo floating on a page does not.
Paper
Photo paper, not plain office paper. Plain paper absorbs ink, so the image spreads slightly, the colours shift and the surface will not survive being handled or written on.
Matte or semi-matte is the safer choice where a specification mentions finish at all, and it is required by some - India among them. Gloss reflects under a scanner, which matters when the office scans your print into a file.
Where an authority asks for something written on the back - Canada, Australia - proper photo paper also takes a pen without pressing through onto the image.
Check the print with a ruler
This takes ten seconds and catches everything above. Measure the printed photo: it should be exactly 35 by 45 mm, or whatever your document specifies. If it is 34 by 44, scaling was on.
Measuring the head is worth doing too - chin to crown, against the band for your document. A ruler with millimetre markings is all it needs.
Cutting
Cut on the marks, not around the image. A guillotine or a craft knife against a steel rule gives a straight edge; scissors rarely do. Leave no white border unless the specification asks for one, and do not round the corners.
IDShot generates sheets at true size for 4 by 6, 5 by 7, A4 and Letter with cut marks, so the only thing left to get right is the scaling setting in the print dialog.