If you want to add to the size of the array, I would use the build array function (option click on it and choose concatenate). This will add the data to the end of the array. If you need to enter data in the middle of the array then you need to use the insert into array function.
If you need to change data fast, then you should pre-allocate the array by using the initialize function and then in your data generating loop, use the replace array subset function. This is better at using memory and faster (although every 20 seconds is pretty slow).
Rob
