Due to the way VST graphics are implemented in the most commonly-used development libraries, resizable UIs are difficult to create, requiring the developer to make separate images for each size.
Most vendors don't bother, but I think size/scale options are going to gradually become a more-or-less standard feature. FabFilter, Voxengo and Native Instruments are three that come to mind that offer 2 or more (fixed) size options. Very few are
continuously resizable, e.g. Valhalla and some Meldaproductions.
It would seem that the more complex the UI the less likely you'll ever see size options. However, Urs Heckmann has listed graphical scalability as one of the expected enhancements in Zebra3, and that's about as complex as plugin UIs get.