I do not know about the standard, but I have been making them customized for the recipient. Some are a bit longer and/or wider than others..... I am making them big enough so they can use them when they want to lay on the couch and be covered chin to toes, lol.
I think it also depends on the quilt block design, some I am thinking about making need a specific amount of blocks to complete the design and so they may come out a little bigger, also.