If you open the AHBOT.XML in notepad you can make sense of the code even if you have no idea what your looking at. I believe the bit you need is as below:
<SellItemOnAhAction Category="TradeGoods" SubCategoryType="WoWItemTradeGoodsClass" SubCategory="MetalStone" UseCategory="True" RunTime="_24_Hours" AmountType="Everything" ItemID="0" MinBuyout="0g10s0c" MaxBuyout="20g0s0c" StackSize="20" IgnoreStackSizeBelow="1" Amount="10" BidPrecent="95" UndercutPrecent="0.0001" AutoFindAh="True" PostPartialStacks="True" PostIfBelowMinBuyout="True" Location="0, 0, 0" />
Just change the the "StackSize" to 200. You can make one just for Blackrock ore if you wanted using the Item ID.