ID card photocopy size: what offices actually expect
85.6 × 54 mm. That is ID-1, the international standard, and a CNIC, a driving licence, a passport card and a bank card are all this size. A copy printed at 100% should measure the same as the card itself.
The ID card tool places both sides at exactly that size on one A4 sheet. ID card scan
Why the size is checked at all
Somebody comparing a copy against an original works faster when they are the same size. It also makes tampering easier to spot: a card scaled up or down has to be measured to be checked, and most people simply do not bother, which is exactly the gap a forgery relies on.
What goes wrong on a photocopier
Two things. Auto-fit, which scales the card up to fill the glass and produces a copy nearly twice life size. And the lid left open, which gives a black page around the card and uses a great deal of toner for no benefit.
When not to force the standard size
Not every card is ID-1. Some student, staff and travel cards are their own shape, and forcing them into an 85.6 × 54 frame stretches the picture into something visibly wrong. Place those at the shape they actually are.
Both sides, one sheet
Almost every office that asks for a card copy wants both sides, and almost every one prefers them on one page. Two separate sheets is two things to file and one to lose.