Each size and colour is its own item
A style in three colours and five sizes is fifteen items, each with its own code, category, brand and stock figure. Group them by brand and category and the style still reports as a style.
Being straight about it: there is no grid that holds sizes across one product. That has a real cost in setup effort, and a real benefit in that every size has its own figure, its own reorder level and its own sales history.