I found a very useful tip for changing the settings of the Alt-Tab previews in Windows Vista. This is the tradition windows selection shortcut, rather than the new 3D version that is accessed with the Windows Key and Tab.

It does involve editing the Registry (Windows Key and R, type regedit and press enter) but I've had no ill effects from adding the Key and individual settings. Sadly all the settings are in hexidecimal rather than ordinary decimal so they are a little hard to decipher.

Thankfully though, the people over at CyberNotes have created a short list of suggested thumbnail sizes depending on your screen resolution.
1024×768: MaxThumbSizePx = 00000078
1280×1024: MaxThumbSizePx = 0000009E
1680×1050: MaxThumbSizePx = 000000d2
1920×1200: MaxThumbSizePx = 000000f5

As ever though, I'd recommend being sure that your files are safely backed up before editing the registry.