The end of this discussion gets to the heart of the matter -- what does the user want? If the user wants bigger characters, buy a bigger monitor or reduce the resolution. I have some points that I always consider:

1) Is there anything really worth resizing? As with the MessageBox() display, there's not much to do except increase the "white" space or zoom up the font -- but that's non-standard behaviour. So, for small modal forms that offer information, condolences on an error, or user choice of action there doesn't seem to be a good reason for scalability. Refer to the numerous forms in the Windows 'Settings', they show the choices, but do not allow user resizing.

2) Need for geography on the display: TreeViews, grids, list boxes, edit boxes are all high priority in my thinking. For heavily used forms, it is worthwhile to record and reinstate the last settings for each user, so they are involved in the presentation of the data -- like they want it. It is beyond just resizing, but allowing the re-ordering or omitting of columns is also a nice touch.

3) Relative object position is critical to keeping the user oriented to the task(s) at hand. Esthetic appearance can be aided by some standards for margin ( e.g. 5 or 10 pixels ) as well as standards for height ( and in some cases width ).

4) I have noticed the 'Corner.bmp' in the lower right of Windows forms that are actually re-sizable, and so I have added this as a visual aid to users, allowing them to immediately know that the form is scalable beyond just the Minimize / Maximize buttons in the form's caption area.

5) On some forms with multiple priority #1 objects for geographic space, analyze the current screen resolution and if the user has not already recorded choices, automatically gobble up a bit more room, e.g. 400 x 300 on a 640 x 480 system, and 600 x 450 on an 800 x 600 system. This allows the user to see relatively more the higher the resolution they are running.

6) Finally, don't overlook MinHeight and MinWidth to maintain the minimum legible form size for the data that must be presented.

Good topic!