Unfortunately the per post signature options has yet come into the setup, Xenforo is a pretty new setup in development so alot of the things they considered to be bloat has been left out so it can be added through either a modification or upon large amounts of requests to be added in later versions.
If you are concerned about the height of your signature , maybe a small compromise ( although not ideal ) is to wrap it in a spoiler....this will then make your signature collapsed by default.
You could always write one line in your signature like "Take a look at my Setup", then spoiler the rest as a collapsed button .
To add a spoiler simply edit your signature and wrap your signature in the following BBcode :
Code:
[spoiler] Your signature here [/spoiler]
Or you can click the little "insert" button in the editor options