-Xms size in bytes
Sets the initial size of the Java heap. 
The default size is 2097152 (2MB). 
The values must be a multiple of, and greater than, 1024 bytes (1KB).
(The -server flag increases the default size to 32M.)
-Xmn size in bytes
Sets the initial Java heap size for the Eden generation. 
The default value is 640K. 
(The -server flag increases the default size to 2M.)
-Xmx size in bytes
Sets the maximum size to which the Java heap can grow. 
The default size is 64M. 
(The -server flag increases the default size to 128M.) 
The maximum heap limit is about 2 GB (2048MB).