If a plate's thrown on a potter's wheel, it's going to be round unless the shape is altered after it's been removed from the wheel. If it's made from a slab of clay, it can be any shape. If it's made by slip-casting in a plaster mold, as is most commercial dinnerware, it can be any shape you can make a mold of.

In my own pottery, I do 99% of my work on the potter's wheel. My plates are round. My wife does 99% of her work using slabs. Her plates are often square or rectangular. Many of her plates are intended for sushi where rectangular is the norm.