Edit your eclipse.ini file and add/change -vm option and then start eclipse.exe
-vm D:/work/Java/jdk1.6.0_13/bin/javaw.exe -showsplash org.eclipse.platform --launcher.XXMaxPermSize 256M -framework plugins\org.eclipse.osgi_3.4.3.R34x_v20081215-1030.jar -vmargs -Dosgi.requiredJavaVersion=1.5 -Xms40m -Xmx512m
You can find more information about these settings on stackoverflow