<CATEGORY name="Big Huge" key="Big Huge" desc="Big Huge">
<DATA>100</DATA>

Changing the 100 into 140 makes the Big Huge maps twice its normal size.

When I set it higher the ingame selection window gets fixed on a default place on the map which makes selecting anything impossible.

What I've finally settled on is the following (excerpted from my personal rules.xml, affecting my in game setup menus for skirmishes)

<CATEGORY name="Tiny (Size=40)" key="Tiny" desc="Tiny">
<DATA>40</DATA>
</CATEGORY>
<CATEGORY name="Arena (Size=50)" key="Arena" desc="Arena">
<DATA>50</DATA>
</CATEGORY>
<CATEGORY name="Small (Size=60)" key="Medium" desc="Medium">
<DATA>60</DATA>
</CATEGORY>
<CATEGORY name="Standard (Size=80)" key="Standard" desc="Standard">
<DATA>80</DATA>
</CATEGORY>
<CATEGORY name="Large Size=100" key="Large" desc="Large">
<DATA>100</DATA>
</CATEGORY>
<CATEGORY name="Huge (No water maps)" key="Huge" desc="Huge">
<DATA>120</DATA>
</CATEGORY>
<CATEGORY name="Big Huge (Himalayas Only)" key="Big Huge" desc="Big Huge">
<DATA>140</DATA>

Note that my experimentation has revealed most water maps have issues with anything over 120 (crashes were common at 140) - nut usually they handle the 120 just fine. Land maps don't do well over 140, you often end up with dead spots along the sides and corners of the map where you cannot select anything.