How much is "1 medium grated apple"?

I guess you mean how big of an apple to use.. I'd say a medium sized apple would be about the size of an average woman's fist. grate it all but the core. Use that as a reference for size, and if you think your apple's too big, grate only 3/4ths of it. Besides, the first time you give the recipe a go, you'll know how to adjust the amount, if you want more or less. So don't stress about it :)