As I mentioned before, deciding card dimensions was a difficult task due to a number of reasons:
You have to be sure that the card is big enough to fit any text, images or other visuals on them
Typically, cards in a card game need to be different sizes to help players tell which is which
They need to be ergonomic: not to big or small to handle, but pleasant to hold
In a single team meeting, my teammate and I decided what size the 'mission cards' would be. Out of all the cards in my game, these are the most important because of their quantity and their contribution to the gameplay.
Since the prototype mission cards were only demonstrative, we needed to scale down the size of the mission cards which will be used in the game. The prototype versions were much, much too big and had on them a lot of negative space, two problems that the actual mission cards would eliminate.
Since the actual mission cards would still need to have the same specifications on them (such as the mission text and the dice roll to complete) we needed to come up with a size which both afforded almost everything on the original cards but was considerably smaller and more ergonomic.
We then experimented with some rudimentary card sizes by putting them through these dimensional lenses, and eventually came up with the perfect mission card size which every playable mission card would be:
13cm (Height) X 8cm (Width).
The photograph above is the master template for all mission cards, created during that team meeting. All mission cards will follow this dimension, and have displayed on them the specifications mentioned previously.