You can change it by editing Info.plist file which is by default located under /Applications/IntelliJ IDEA X.X.app/Contents directory (Choose Show Contents in Finder, X.X is the IntelliJ IDEA version number).
Quick way to open this file is to type the following in the Terminal:
open -e /Applications/IntelliJ\ IDEA\ X.X.app/Contents/Info.plist
Note that spaces should be escaped with backslashes.
To change JVM version, find the following section in this file:
Change the version number depending on your needs. Note that IntelliJ IDEA doesn't run well on JDK 1.6 on Mac because of multiple Apple bugs.
You can check the effective JDK version used by IntelliJ IDEA in Help | About dialog.
Heap size and other JVM options are changed via the following section:
<string>-Xms16m -Xmx192m -Xbootclasspath/p:../lib/boot.jar -ea</string>
For IntelliJ IDEA 12 please refer to this answer.