Like the title says. Variables created in the trigger editor can be made into arrays, but the size of the array cannot be changed from 1. Changes made to an array’s size are not saved under any circumstance, as far as I can tell; it always reverts to 1. Needless to say this breaks a great deal of functionality.
Be aware that the array size field is only used for initialization of values in the array. The underlying arrays are dynamically sized and expand as required. Your own initialization script or trigger can be used instead, letting the array size field be left at 1 and allowing use of array indices up to the value defined by the maximum array size global constant (32,768 indices, 8,191 in legacy versions).
constant integer JASS_MAX_ARRAY_SIZE = 32768
The main use for the array size field, if it works, is for complex object types such as player groups, timers, e.t.c. Without that field you may need to use custom script to initialize indexes to such an object.