Auto Index - when checked the node will automatically output all indices.
Array Index - select the index within the array.
Add - use this button to add more rows (indices) to the UI.
0: Value - the first index in the array. Set values for X and Y.
Create a Value 2 Array.
Create a second index (Add button).
Set values for X, Y of 1920, 1080 and 720, 720 for each index.
Open the Composition Settings and, by clicking a dragging its header, make the window persistent.
Connect value2Array.id > composition1.resolution.
Uncheck Auto Index on the Value 2 Array.
Set the Array Index to a value of 1.
As you toggle the Array Index between values of 0 and 1 the Composition Resolution will change from 1920 x 1080px to 720 x 720px.