Mug Crumbles

I got this cute Mug Crumbles recipe book for Christmas from sister C.  I am going to try one tonight and will see how it goes.  The measurements are really non-specific (I had to look up a "knob of butter"!).  The recipe I'm going to try calls for two apples, skinned and diced with a topping.  What size mug can hold two apples and topping? Sounds delicious.  I'll let you know.
I added an extra of the finished product.  I split it into two large mugs and it looks like it could have fit into one, once cooked.  However, it would've been way too much for one person.  I'll have to try this recipe again, and another too.