Slabs, namely engineered stone slabs vary due to the fact that it's made in a mould. Although moulds do vary, we have slab by slab moulds for quartz slab production lines, and block mould for marble based production lines. In reality the equipment was made to get an even 300 cm x 120 cm in the marble based material, and a 300 x 140 in the quartz.
In the agglomerate marble production lines the most common measure is 300x120, although in truth the block has a few more centimeters, so you'll probably be invoice for 306x123.
In the quartz agglomerate production lines the moulds can vary in size, technically the same production line can make 300x120 and 300x140 slabs. Regarding the exact size of the slabs, some manufacturers invoice 300x140, others 307x124 ... their is not really any limitation to it at the moment. The CE markings on slabs may shed some light on this issue. Nevertheless, be careful if you buy 300x140 and try to re-sale it with an add-on of a few cm, some manufacturers trim the edges with defect ... so before you send the slabs to the final destination make sure you take correct measurements of the whole load.
In recent years some manufacturers have bought bigger production lines and now sell different sizes than the ones I mentioned, however more than 80% of sales are still in these traditional measurements.