Gold bars come in all shapes and size from the very small wafer thing 1 gram 'biscuits' to the heavy 400 ounce ingots, so favored in Hollywood movies. The larger bars are called ingots and are made by poring molten gold into moulds. This is called casting. The smaller bars are made by pressing gold with a hugh multi tonne stamp, much in the same way gold coins are made, and these are usually called biscuits.
All bars of gold should have stamped on them, the weight, the size and the manufacturers stamp. Also a registration number that corresponds with a certificate which should also accompany every bar.
The advantage with the smaller bars is of course convenience. They are easier to buy and sell due to the lower cost involved. Easier to store and transport. The disadvantage is the price. They carry a higher premium over the value of the gold due to the costs involved in manufacture.
The advantage with the larger bars or ingots is the premium is much lower and usually a fraction above the spot price of gold for that day. The disadvantage is that they are more difficult to move around and store due to their weight and can me more difficult to sell as there is much more funds involved in the transaction. Most of the larger ingots are stored in bank vaults around the world.
To buy bars of gold then is dependent largely on the purpose for purchase. If you are interested in storing assets in gold for the long term then the larger bars may be more suitable. If you cannot afford the large bars then perhaps you can look at a smaller bar and gradually build up with smaller 10 ounce, for example, bars which are easier stored and transported.
If price is not a consideration and you like the idea of collecting and owning gold bars the 1 ounce and smaller may fit the bill. The premium is larger and can even stretch to double the value of the gold but over time this will be recouped as the value of gold against the currency increases.
In any event, to buy bars of gold can be a lot of fun and there is nothing quite so wonderful as holding a bar of gold in your hand.