Resizing cards is pretty simple. Hover your cursor to the right edge of the card and when this two-sided arrow ⬅➡ shows up - drag.
Check the GIF below
However, if you are at the edge and don't see the two-sided arrow ⬅➡, it means that the card is at full-screen i.e. maximized. In that case, you have to un-maximize first.
Check this article to learn more on how to make card go full-screen or un-fullscreen.