No, kitchen worktops are not a standard size. Depending on the manufacturer, kitchen worktops can vary in width and depth. Additionally, the thickness of the worktop can also affect the overall dimensions.
If you buy a laminate worktop they tend to be around 2.4m or smaller in length, with a width of around 640mm and a thickness of 40mm. Of course there are variations, for example kitchen islands are around 940mm. The idea being the worktops sit on top of 600mm and 900mm cabinets and have a slight overhang to prevent spills going down the drawer fronts.
Stone and engineered stone worktops tend to arrive in slab form at the fabricators of 3m x 1.2m or thereabouts and are then cut down to size and polished around the edges before being professionally fitted.
Any worktop can be custom cut to size depending on your kitchen layout.