The cap height of most Postscript fonts is between 680 and 700 units, so for the sake of not confusing end users it’s probably best to keep your design within that range. People start to get confused when your twelve points looks like any other font at fourteen.

Thanks, James. Is that the only factor?

In general I leave a talus (internal leading) of less than 1% (and most of that at the top) and fill the rest with top of ascenders to bottom of descenders. And it's good to allow the cap accents to float above the top of the EM.

hhp

A talus? Can you explain anymore, Hrant? Thanks

You leave space within the EM above the top of the ascenders and below the bottom of the descenders, to avoid collisions in the case of flush setting (plus to meet the size expectations of users, as James said). The sum of these two "blank" spaces is the talus. It's not a common term, but "internal leading" is cumbersome and potentially confusing.

Also, it's smart to leave a greater proportion of the talus on top, because ascenders are much more frequent than descenders, hence benefit from the greater room to breathe.

hhp

Right, that makes sense, I shall use the term from now on. What did you mean by 1%? Sorry to be trivial, thanks for your help!

I don't know why I wrote 1%, I wasn't thinking straight.
It's 5%. In an EM of 1000 that's 50 units.

hhp