According to the International Organization for Standardization, a credit card's size must be 8.56 cm wide by 5.398 cm high. ... All credit cards must be .76 mm thick ( or 0.0299 inches).

If the cards were 4 inches wide and 3.5 inches tall, folding them in half would make them 2 inches wide and 3.5 inches tall. That’s pretty close to the size of a standard credit card.

The credit card was just the business card juxtaposed to the size of the standard business card, for ease of use in the wallet. The history of the card depends where you look.