As BlueHazzard said it is a linker option. In the template you have, depending on selected compiler:
sdcc
project.AddLinkerOption(_T("--code-size " + format("%u", SizeCODE)));
keilc51
project.AddLinkerOption(_T("CODE(0x0-" + format("0x%04X", SizeCODE - 1) + ")"));
keilcx51
project.AddLinkerOption(_T("CLASSES (XDATA (X:0x0-X:" + format("0x%04X", SizeXDATA - 1) +
"), HDATA (X:0x0-X:" + format("0x%04X", SizeXDATA - 1) +
"), CODE (C:0x0-C:" + format("0x%04X", SizeCODE - 1) +
"), CONST (C:0x0-C:" + format("0x%04X", SizeCODE - 1) +
"), ECODE (C:0x0-C:" + format("0x%04X", SizeCODE - 1) +
"), HCONST (C:0x0-C:" + format("0x%04X", SizeCODE - 1) + "))"));
iar8051
project.AddLinkerOption(_T("-D_CODE0_END=" + format("0x%04x", SizeCODE - 1)));