Placement is worth more than design. A card at the card reader outperforms a poster on the wall by a wide margin.
The scan happens in a narrow window: the customer is stationary, waiting, and holding a phone. Your job is to be in front of them during that window and nowhere else.
Print at least 4cm across so a phone locks on from 30cm away. Keep good contrast, avoid glossy laminate under a spotlight, and put one line of text above the code so people know what it is before they aim a camera at it.
Two cards in the right two places beat ten scattered around the room.