For gold, bars of acceptable delivery must be of a minimum fineness of 995.0 and a gold content of between 350 and 430 fine ounces with the bar weight expressed in multiples of 0.025 of an ounce - the smallest weight used in the market. Bars are generally close to 400 ounces or 12.5 kilograms.
For silver, bars of acceptable delivery must have a minimum fineness of 999 and a recommended weight between 750 and 1,100 ounces, although bars between 500 and 1,250 ounces will be accepted. Bars generally weigh around 1,000 ounces.
There are additional criteria specified for good delivery status that are set out in detail by the LBMA on their website.