Using setPreferredW/H is HIGHLY discouraged. setPrefferedSize() is also problematic.
Testing for a specific device is usually a mistake, you should test for density or work with constraints.
If you need to do something like this try doing this:
Dimension d = btn.getPreferredSize();
d.setWidth(Math.max(d.getWidth(), Display.getInstance().convertToPixels(10, true)));
btn.setPreferredSize(d);
What I do here is two main things, I use the existing preferred size and never go below it.
I use millimeters instead of pixels in the preferred size which should give me a reasonable approximation across devices.
Notice there is no need for iOS/Android specific code.