Creatures and objects of various sizes involve different amounts of space. Before you used categories as Fine, Tiny, Small, Medium, Large, Huge and Colossal. Those tools help to describe fights and encounters because they show sizes, the spaces that they involve, and areas of impact.
Various sizes also have different uses. Creatures whose size is not Small or Medium require items apt for their body. Those items have different Bulk and usually different Price.
For instance, morningstar for Medium creature cost 1 gp and have 1 Bulk. For Huge creature it costs 4 gp and have 4 Bulk. The table about items of different sizes gives the conversions for Price and Magnitude.
The space that creature involves on grid, present in the table about Size and Reach. It lists also typical reach for creatures of every size. That counts for long and tall creatures, as the most bipeds.
For instance, Fine creature involves 1/2 ft of space with 0 natural reach distance. Tiny creature takes 1 ft with 0 reach. Small creatures involve 2½ ft with 0 reach, while Medium involve 5 ft. According to the size chart, creatures of Fine until Medium use 1 inch diameter circle as base.
Large creature uses 2 inch diameter base.
Size affects also Bulk. It shows size, weight and general awkwardness. Large creature has 2 times the Bulk of Medium creature.
Huge have 4 times that of Medium. Gargantuan have 8 times. Size sets bases for physical skills and natural weapon damage.
Most creatures range of Small until Huge, but other sizes happen. If you expand sizes in one step, the original damage moves in two steps according to the chart. Small character with small weapon doing 1d6 adjust to 1d8. Medium weapon of 2d4 become 1d6. Naturally Colossal creatures do not receive size bonuses to stats, but they simply have higher stats.
Game master should ignore the chart about size increase for stats, unless you grow them.