If you search on automatic for size, the search results are shown by the size GG used to identify the number. If you try to change a 2 byte memory space to 4, you are making the app 2 bytes larger or writing over the next 2 byte memory space. There is no reason, in fact it would be very harmful, to change the byte size, that's why the other sizes are grayed out.
If you want to find the same number, but at 4 bytes, you'll have to select 4 bytes when you initiate the search. If it doesn't find it at 4 bytes and shows it as 2, that's the number of bytes it is stored in. It is showing you the size for informational purposes only.
I hope this makes sense!