Based on the specific thing you ask for.
For example, in XBMC, you say "Help TV commands"
Then in very large font, TV Commands would show up.
In the meantime, here's a nice work-around I just found.
Open up your browser on the HTPC (google chrome, in my case).
Generate the help menu off of voxcommando - this will open in your browser of your choice. Use cntrl-+ to make the font larger, and then maximize the screen.
Close out the browser - the next time that it launches, it will launch the way that you set it.
You can use "Page up" "page down" to quickly scroll up and down in the browser.....