I absolutely understand it's easy to forget some parts, and there could be more practices to go more gradually, as well as more user experience features if we had more funding. The app helped many people learn, and we're well aware that it can be improved and how. We've started work on a complete remake from scratch, though the lack of funding for this kind of free and open-source project makes it challenging to achieve.
To answer the specific case of the for loop you mention, I would invite you to re-read the lesson dedicated to for loops, where this is introduced ( https://gdquest.github.io/learn-gdscript/#course/lesson-18-for-loops/lesson.tres ).
When you write a for loop, you define a temporary variable to which the computer will assign individual values from the array. It's something we introduce in the lesson linked above. So when writing for size in rectangle_sizes, the size bit is a variable that we, the developers, create. You could name it however you'd like, for example, for current_rectangle_size in rectangle_sizes.
In each loop iteration, the loop variable will receive one value extracted from the rectangle_sizes array.