Hi,

Here is the answer,

The preferred size of a component is the minimum component size that will allow the component to display normally.

When frames or containers are designed using layout managers, each layout manager imposes certain rules on how the components within the container must be placed and sized. Components cannot impose their required size on the layout manager hence they use the "preferred size" attribute to suggest to the layout manager of what wud be the ideal size/location to display them.

Thanks.