First, I know it depends on the content. And after reading through ux.se, blogs and observing websites; the results are quite less.
What I've found so far:
- Text is readable at best ~100 characters (like ux.se), but people prefer 50-75 (like popular websites like facebook, google)
- Cards should be as small as possible
Those 2 are already tricky. Google+ shows between 50-80 characters on the card, mostly depending on the screen size (they have three different card sizes). They use 13px Arial, which is quite small. At least according this article from smashingmagazine proposes to use at least 16px. Does big font size even coincide with card patterns?
Luckily in my position I usually don't have long text. It's a business software and we use a lot of data fields, mail, name, etc. And that's the problem: data field length is unpredictable. How long can a mail address be? Should the card size be designed to fit for 99% of all occurences? For the 1% use truncation?
And what about responsiveness?
Should the card:
- keep always the same width (as described in Material Design)
- adapt to screen-size (min-width and max-width, depending on available space and how the rest of the UI adapts)
- using an amount of fixed sizes like google+ (360px, 440px, 520px)
Are there any additional guidelines, best practices, thoughts?