Hi ak,
The best way is to use "Initialize Array" from the array palette. You have to wire an element to define the array type. If you know the array size, you can wire it, if you don't know it, you have to append new data to your array with "Build Array" function (also in the array palette). Build array is a resizable function.
Nevertheless it is better to fix the array size once for all because if the array size is getting too big, labview allocate entire new space for it.
Cheers.