If it's true that memory of jass arrays is allocated in power of 2 according to the highest index currently used, then it's the same with sized arrays.
Since sized arrays are nothing but arrays with pseudo size (you can go out of range), it is likely to depend the highest struct instance used.
Check the outputwar3map.j to see how the jass code is generated.
Now, frankly you don't really have to worry how much memory used in jass, especially for this. (except avoidable memory handle leaks)