I should have done this in the first bloody place.
OK...I'm creating a card based game, the cards are all poker sized.
What is the best font size to display on them to ensure easy readability for the players?
Font is everything - then the right size for it.
Don't laugh - if you have friends that wear eyeglasses - have them see if they can read the test cards without their eyeglasses on! Many people that wear eyeglasses take them off to play card games so no one can see the reflection of their cards on their lenses!
So I got my test cards. The 8pt font turned out fine and a little better than I expected honestly. In fact I printed a test card with 7pt and while it was smaller than I would like I could easily read it.
I used publisher and 8pt Calibri font. I converted the images to 300 dpi jpg & png files. Both worked the same as far as I can tell.