That race editor wasn't made for LH, so it won't work right. Making a custom faction is easy you can use the in game editor. Making a custom Race is not that hard unless you also want a custom modeltype so your unit has custom clothing and looks. That's is very very time consuming. You don't so much add a new race as edit 90% of the xml to include it. Look at some mods that contain races such as my Additional Traits mod or Children of the Storm.

<ModelScale>1</ModelScale> Will increase the size of a unit.

and some fallen male units have a different body type that is larger and more muscular such as

<UnitModelType>FallenMale</UnitModelType>

However no you can't make a unit fatter through xml.

I don't think you can disable a race. You can disable a faction though by adding  

<HideInSetupLists>1</HideInSetupLists> 

to the faction xml.