Unfortunately to measure the width (or the girth) of a gray whale it needs to be dead. Sometimes whales end up beached and this provides an opportunity for measurement. Only 5 gray whales have ever been measured in this way. Male grey whales are estimated to have a girth of between 20-30 feet. Females are even larger.

I'm not sure, but they can get up to about fifty feet long and weigh about 36 tons. When you get to high school (or maybe junior high), you will learn the math you would need to calculate the width based on what I have just said.