Enter a terminal, and if using gnome, type;
Code:
gedit ~/.config/cairo-dock/current_theme/cairo-dock.conf
If KDE substitute kwrite for gedit. Somewhere in that file is the setting you changed - sorry, but there's no easy way to guess what it is. Look for something obvious, and if you can't find it, post back, and we'll try to copy some sensible defaults over it.
[ETA on second thoughts I have no idea where KDE keeps its cairo-dock settings, so the substitution may not work - hopefully you're using gnome]
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