Three sizes for literature racks and countertop holders. Width is the only dimension that decides whether the card is displayed at all. A few millimeters too wide and it does not go in the slot. Filter by the rack you are filling.

4″ × 9″, broad distribution across many racks.

3.5″ × 8.5″, slim racks and countertop holders.

Your dimensions, a bespoke display or matched collateral.
The 4 x 9 is what most racks are actually built around, so it fits essentially everywhere without checking. For material distributed across many locations that reliability beats any distinguishing dimension. The narrower 3.5 x 8.5 suits slim racks and countertop holders, and sits noticeably smaller in a standard slot, so choose it when you know the displays rather than for general distribution.
Design for the top third. Cards sit behind one another in a rack, so often only the top two or three inches are visible. Whatever identifies the card belongs in that band, and everything else is read only once it is in someone's hand. Stock matters more than it looks: below about 300gsm an unsupported card bows forward in the slot within days and stops facing the room.