Like Leeann I have the Sizzix strip die ones - little letters that are a great size for cards. I have the Typeset one as the style is quite versatile but there are lots of different fonts. They're here
on the Sizzix site (some have both upper and lower case, some are just upper case with slightly bigger letters).
I've attached a card (5" square) and a postcard (4" x 6") with Typeset lettering on them so you can see how the size works out.