Essentially a repost of http://forum.lxde.org/viewtopic.php?f=21&t=1423 (yes, I know that was not the place to submit feature requests)
I took a look at the code in lxpanel-0.5.5-2.fc13.src.rpm, but am not set up for C development. It appears that allowing the user to control the font size in lxpanel will require:
1. modification of the Panel structure (panel.h) to accommodate the new parameter
2. some code to read fontsize (panel_parse_global() in panel.c)
3. modification of the code that currently sets font_desc (panel_draw_label_text() in panel.c) to bypass this code block if fontsize is explicitly given.
It would also require adding this feature to the configuration dialog. We would not allow a parameter that can only be adjusted by editing the configuration file.
And I probably missed a few places that need work; my first attempt to patch panel.c and panel.h did not produce the desired result.