I'm no expert, but here's what I've learned from my experiences regarding this issue...
Each PowerGadget saves a personalization file that specifies the size and placement of the gadget. These can get corrupt or messed up, and when this occurs the gadget won't open the same way it was closed last.
These personalization files reside in C:\Users\yourusername\AppData\Roaming\Software FX, Inc\PowerGadgets\Personalization. You can delete the entire Personalization folder if you want, and everything will get reset to default, or you can find the personalization file associated with the GUID of the gadget and just remove it. I believe this is true for all versions of Windows.
Alternatively, I've found that if you open the gadget in PowerGaget Creator, go to Edit->Desktop Settings->Layout, specify a location/width/height, uncheck Personalizable and set Floating in Edit->Desktop Settings, the gadget will always remember these settings and the personalization file won't be written. This has given me the best luck with guaranteeing the size and placement of the gadget, however the horizontal/vertical offsets can only be set at 1024 maximum using this interface. If you need to exceed 1024, you'll need to open the gadget in Notepad or Notepad++ and edit these numbers manually.