You can do it using the underlying java table, with some undocumented options (well, they are documented, just not in Mathworks documentation!), but not in Matlab itself.
As ImageAnalyst mentions, AutoWidth is just for sizing individual columns and it only works when you actually run the GUI, the table representation within GUIDE is pretty static apart from updating with the column names, etc.
This is a restriction of uitable in general, rather than anything specific to GUIDE though - the same behaviour occurs when just creating a uitable yourself (in terms of how auto column width works and having to use java to fill the full table width).
It is a very annoying feature because badly sized tables do look rather ugly in a UI and the possibility for scrollbars in a table can add extra complications too.
