Can I reset app windows to default size/position?
Window size preferences are generally stored in the .plist files found in
/HD/Users/YourName/Library/Preferences

You may need to enable hidden files by using: 
defaults write com.apple.finder AppleShowAllFiles YES  in command line, followed by a relaunch of Finder.

As an example, for Finder the file is called com.apple.finder.plist and the window state information is under the key WindowState for each different type of finder window (e.g. standard folder, searchview...) Other applications have similar naming (ctrl+F is your friend here!), e.g. the Fontbook app uses the NSWindowFrame key. 

You should be able to safely remove the window state information as it get auto read from defaults database anyway.



Having said all that, this doesn't appear to work for a few applications such as Maps and Mail (which is what you want unfortunately). But hey, it's a start.

(Also apologies for the lack of codeblocks, they are refusing to show up for some reason)