Inconsistent naming of size/length methods
For [strings](http://doc.kaitai.io/user_guide.html#_strings) and [byte arrays](http://doc.kaitai.io/user_guide.html#_byte_arrays) it's called `length`, but for [custom arrays](http://doc.kaitai.io/user_guide.html#_array_types) (created with `repeat`) it's called `size`. Is there a reason for the different names? To me it seems like `length` and `size` are do the same thing (just for different types), so they should be named the same as well.