The actually used area is defined in heap.c, so you have to recompile it.
Either recompiling the complete ansi library (lib\hc08c\hc08_lib.mcp),
or by adding lib\hc08c\src\heap.c to your project, move it in the linker tab before the ansi library and build it (not tried, but can imagine that this causes some linker warnings tough)
Daniel