Increasing Java's memory allocation

You can increase the amount of memory allocated to your Minecraft instance by going to the Java tab, in the Settings dialog, and altering the values in the Memory section.

In general, values of 1024 MB minimum, 2048 MB maximum and 128 MB PermGen are adequate for modded instances, though you may need to increase this for larger instances. If your using 32 bit java, please do note the the max ram can be no higher than 1500MB

If you get a 'java.lang.OutOfMemoryError' exception when you try to run an instance, you probably need to increase the amount of memory allocated to it.

For more information about this page, checkout java settings