There is no command to enlarge the heap. It will grow when you request more memory.
Give us more details. I just wrote a mini app in VS.NET 2003:
int
main()
{
BYTE *r1, *r2;
r1 = new BYTE[64000];
_ASSERT(r1!=NULL);
r2 = new BYTE[64000];
_ASSERT(r2!=NULL);
return 0;
}
Works without a problem.