What does "credit card sized" mean?

Solution 1:

The term "credit card sized" generally just refers to the two longest dimensions. It is not normally used to indicate that the object is also the same thickness as a credit card.

Solution 2:

Credit card sized simply means it is approximately the size of a credit card. In this case, the usage is somewhat unfortunate as it does not approximate the thickness very well. There is no idiomatic usage of the term.

Solution 3:

Context is everything.

In this context:

it is clear that "credit-card sized" refers to the size of the circuit board, rather than the size of the board + the components soldered onto it.