In general, larger text sizes work better than smaller. ‘Fill in’ is a term used to describe bridging between the open areas of a character, or between two characters, which affects the legibility of the text and overall appearance. However, copy sizes that are too large can present problems with ‘air entrapment’ on heavily coated or laminated stocks that can cause the foil to adhere improperly or not adhere at all. As a general rule, you can utilize smaller type sizes on coated stocks versus softer, more coarse or textured uncoated stocks. The hot stamping die will deboss into the softer, uncoated stock much more easily and can potentially cause ‘fill in’.
Finally, because typefaces generally appear bolder when foiled, don’t track or kern too tightly. It is a good rule of thumb to set type loosely and with more leading than you might ordinarily consider. With embossing, if the letters are close to each other, create more space (kerning) to allow for the bevel (the edge of an embossed or debossed area made to a specific angle to the paper plane).
As with all situations, it is always best to communicate with your foil stamper first to help you determine the type parameters on your specific job and stock.