When using a grid system, which property typically defines the size of a column?

The property that typically defines the size of a column in a grid system is the column span. This property determines how many columns within the grid the element will occupy. For example, if a column span is set to 2, the element will extend across two columns of the grid. This is crucial in responsive design, as it allows for dynamic layouts that can adjust based on the content and screen size.

In contrast, other options like width percentage refer more to the specific pixel or percentage width of a column rather than the actual grid structure, which is vital for grid-based layouts. Similarly, container size pertains to the overall dimensions of the containing element but does not denote individual column width directly. Flex-grow is a property used primarily in flexbox layouts that allows items to grow and fill space but is not typically relevant in defining grid column sizes. Understanding the concept of column span is essential when creating or modifying layouts within a grid, as it provides direct control over how grid items are arranged and how space is utilized.