Almost all dialog boxes and many other windows are of fixed width and can not be maximized from its user interface. The developer doesn't want you to maximize it, so they either hide the maximize button or disable it.
As I mentioned, this is a front end locking only and the window or program will still respect your instruction if somehow you manage to reach them. We will see two methods for the tweak.
Maximize Programs through Task Manager
Any program can be minimized or maximized from Windows Task Manager. We will take example of calculator app which doesn't support this functionality from its interface.
Step 1: Open Calculator program. You can type calc in run box to open it. 'Run Box' itself can be another example of this case.
Step 2: Open Task Manager and go to Processes tab if using Windows 8. Or, go to Applications tab if using Windows 7 or earlier version.
Step 3: Expand the Windows Calculator tree and then right click on Calculator. Click the Maximize option on context menu. The window will be maximized.
Maximize Programs using AutoHotKey
WinMaximize command any program or window can be maximized from within AutoHotKey script. It doesn't matter if the maximize button is disabled in the program. You can read the full solution on this superuser post.