Resizing the actual buffer at runtime is not recommended. It would require reallocation of memory, which is pretty slow. Resetting the device would likely slow the application down a bit, so I'd say it wouldn't provide smooth resizing.

If you're looking for smooth resizes, I'd recommend you create a buffer big enough for the window's max size, and only use a portion of it when rendering. This would waste a bit of memory, but would provide smooth rendering through resizing.

The Present call can accept two RECTs for the source and destination areas. These can be used to only use a portion of the backbuffer.

Hope this helps.

Thats typically why games do not allow you to resize. I would recommend not allowing resizing at all. However, you should allow the player to fullscreen. In which case, you would need to recreate the device and textures. In this case, a gross slow-down (2-3 seconds, depending on how many/much textures you need to reload) is fine.