How to change the android emulator RAM size from the command line?

I want to edit or change the ram size while creating the android emulator from command line.

EX: While creating the emulator it's taking default ram size(Android SDK 4.0.3) 512 MB But I want to increase it to 768MB or decrease it to 256MB.

I want to change only Ram size. Because there is an option to change the Ram size.

 Do you wish to create a custom hardware profile [no] Yes

If you entered yes, we need to provide so many things.