How can I enlarge the Mac’s preview window?
This is answered over here, but basically you'll need to edit the "QLPreviewHeight" and "QLPreviewWidth" values in the Info.plist file inside the QuickLook plugin that handles images (you'll need select the plugin and choose "Show Package Contents" to get at the plist).

You can see your list of plugins with the command: qlmanage -m plugins

If qlmanage shows that "public.images" are handled by the built-in plugin (/System/Library/QuickLook/Image.qlgenerator), you shouldn't need to adjust dimensions since the window opens to the image's size by default. However if you do need to make changes, just copy the plugin into your directory (~/Library/QuickLook) and then edit it with Apple's XCode (downloadable from the App Store).