Why the wood slabs standard size is 244cm X 122cm? not like 240cm X 120cm or anything else?

You need to learn the words for whatever business you are in. "Wood slabs" are called plywood and they are 8 feet by 4 feet. Standard thicknesses are measured in eighths of an inch and the number of plies is an odd integer. Recently they have started using other materials such as wood sawdust or flakes, and something called "medium density fiberboard", still using the same standard sizes.