Potatoes

It irritates me when a recipe says "3 medium potatoes." Or whatever...but is there 'guidelines' for a medium potato? There's baby reds and baby yukon golds. Then there's the normal reds and yukons. Then there's russets and jumbo russets...the normal reds are smaller than the russets. So right there are 4 sizes. What is medium in all of that?

I tend to need things to be pretty exact when I try a recipe. Im nervous when the recipe is loosely written...like the 'medium' potato thing...