they should have transferred over if you use(d) Migration Assistant.
Anyway, here (in your $HOME folder)..........
Library/Preferences/com.apple.print.custompapers.plist
Although it will be a binary plist so you can't read it (unless you convert to text via the 'plutil -convert xml1' terminal utility)
<key>7x10 Card</key>
<dict>
<key>bottom</key>
<real>9</real>
<key>custom</key>
<true/>
<key>height</key>
<real>504</real>
<key>id</key>
<string>7x10 Card</string>
<key>left</key>
<real>9</real>
<key>name</key>
<string>7x10 Card</string>
<key>printer</key>
<string> </string>
<key>right</key>
<real>9</real>
<key>top</key>
<real>9</real>
<key>width</key>
<real>720</real>
</dict>