From here:
-Xmn : the size of the heap for the young generation
Young generation represents all the objects which have a short life of time. Young generation objects are in a specific location into the heap, where the garbage collector will pass often. All new objects are created into the young generation region (called “eden”). When an object survive is still “alive” after more than 2-3 gc cleaning, then it will be swap has an “old generation” : they are “survivor”.
And a more “official” source from IBM:
-Xmn
Sets the initial and maximum size of the new (nursery) heap to the specified value when using -Xgcpolicy:gencon. Equivalent to setting both -Xmns and -Xmnx. If you set either -Xmns or -Xmnx, you cannot set -Xmn. If you attempt to set -Xmn with either -Xmns or -Xmnx, the VM will not start, returning an error. By default, -Xmn is selected internally according to your system’s capability. You can use the -verbose:sizes option to find out the values that the VM is currently using.