Yes our evil plan is to have as standard five smilies for each line of code in Zen ... it's going to be a tough job but we have the graphics gurus to achieve the target.
Which reminds, must get back to the one I'm working on ...
Regarding the line-height and smiley size discussion, I've created a helpful graphic:
The "line-height" includes the actual height of the text, plus any spacing between one line of text and the next. So, in theory, a smiley should be able to be up to 21px in size without disrupting the flow of the lines or changing the line-height.
The weird thing is that Firebug shows the line height as 20.5333 px, and if I use Firebug to increase the size of a smiley to 21px, it actually does disrupt the lines. Weird...
In IPB, you can have multiple smilie sets, for different seasons/colored themes etc, it is a really nice feature I would love to see something similar in xenforo.