Structure blocks can store areas larger than 48x48:

As you can see in the picture "Structure_1", there is a structure block whose size is 48x48 (as you can see in the picture "Structure_2"). So the gold block in the upper right corner should not be saved. But if you go to the structure block, then change the input fields with the size from 48x48 to 49x49 and then click save without leaving the structure block, then the gold block will be saved. If you then open the file and go to the entry with the structure, you can see that firstly the new size has been saved and secondly that there is an entry with a gold block. (seen in picture"Structure_3").And then when you try to load this structure, the structure block only gives the message "Structure mincratf:structure position prepared" whenever you try to load.