setMinimumSize was added to [font=courier new]Window[/font] class in Java 6, but there's still no [font=courier new]setMaximumSize[/font] method [:(]

There's no API in UserPreferences class to manage additional preferences, and nothing in the user interface for that. At the moment, this can be done only by subclassing this class in a derived version of Sweet Home 3D.

Managing the size of the panel isn't difficult, just call [font=courier new]dialog.setResizable(true)[/font]. I checked in [font=courier new]PhotoPanel[/font] class and the photo component at the top will be enlarged accordingly. But the space at the left and right of the components at the bottom may look uselessly large if you don't manage their layout differently. It's probably the reason why I didn't make the dialog resizable.
An other option could be to add some zoom -/+ buttons to view details in the photo component, like in the background image import wizard.