A standard gold lot is one hundred troy ounces of XAUUSD, which means a one dollar move in the gold price produces approximately one hundred dollars of profit or loss. Mini lots of 0.10 control ten ounces and micro lots of 0.01 control a single ounce, giving roughly ten dollars and one dollar per dollar of gold movement respectively.
Position sizing on gold is a division, not a preference. Divide the money you are willing to lose on the trade by the distance in dollars between your entry and your invalidation level to get the number of ounces, then divide by one hundred to get the lot size. Risking two hundred pounds with a four dollar stop gives fifty ounces, or half a standard lot.
Contract size, minimum volume and margin requirement are all broker-set values that appear in the MetaTrader symbol specification. A minority of brokers define a gold lot as ten ounces instead of one hundred, so checking the specification before the first trade prevents an accidental tenfold error in exposure.