I'm plagued by this problem in Windows XP. Win2k and Win 95 never had a problem; how could they break this (and a thousand other things) in XP and higher??

I've had luck retaining the size by opening Explorer, sizing it, then closing it by clicking the File menu then Close. Open Explorer immediately again and it may retain the size. Close again using File > Close.

But will it retain that setting after reboot? If you close Explorer with the X button or Alt-F4 then, soon after, reboot, it seems to forget your size change. You may have better luck by deliberately closing using the File menu, then reopening to be sure it understood you.

Once you've done the above and the sizing has survived a reboot, the sizing will likely stay that way. All this also applies to View preferences (List, Details, Icons, etc), which often revert to the default that you've set for all folders. I intentionally set List for all, then Icons for Control Panel, but it reverts to List until I do the above.

I'm an IT professional, I should not have to resort to the superstition I've written above, but the disgracefully broken UI in Windows XP (most likely carried over to Vista) has demanded it. Good luck.