Many people use the term longarm to encompass any size quilting only machine. They will even refer to the 9" domestic machines as a longarm if they are sitting on a frame. Mid-arm to most is any of the 9-18" machines that can be used on a machine quilting frame but are not necessarily permanently fixed onto a quilt frame like a true longarm.
Some people call the 10-15" machines short-arms & 16-18" midarms. Knowing the different machines & moddels is the only real way of determining how large the throat. Even then you will occasionally run into a machine that has been stretched or a converted industrial machine that doesn't fit in any of the short, mid, or longarm categories.