Untested, taken from XPize Settings (Regshot).

32 pixels

HKCU\Control Panel\Desktop\WindowMetrics\IconSpacing: "-1275"
HKCU\Control Panel\Desktop\WindowMetrics\IconVerticalspacing: "-1125"
HKCU\Control Panel\Desktop\WindowMetrics\Shell Icon Size: "32"

48pixels

HKCU\Control Panel\Desktop\WindowMetrics\IconSpacing: "-1371"
HKCU\Control Panel\Desktop\WindowMetrics\IconVerticalspacing: "-1371"
HKCU\Control Panel\Desktop\WindowMetrics\Shell Icon Size: "48"

Edited June 23, 200620 yr by Camarade_Tux

HKCU\Control Panel\Desktop\WindowMetrics\IconSpacing: "-1275"

[...]

Oh, oh, WindowMetrics!

I had a long and fierce battle with it. And lost.

Impossible to load any settings in that key.

They were always overwritten by the theme manager.

To win the war I had to use the nuclear option: save a custom theme with my settings and have the theme manager import it.

@dhedges, my settings were only about the windows' title bar's height. I'm not saying that your settings will have to be dealed with the same way, but be prepare to fight and don't be disapointed if it doesn't work that easy.