The Optimum Legend Size

From Facebook:

Hey all,
I am trying to create some custom legends in Photoshop for some of my playbacks. When I import them, they are much smaller than the playback button, and they dont scale to edges of the playback button. Is there a secret to getting this to work?
Thanks!

Some example pictures, quickly made with Paint (the preview resizes the pictures - click on them to see in their original size):

150 x 150 px

200 x 100 px

600 x 600 px

800 x 400 px

I'd make them e.g. 200 x 100 px. The result depends from display settings, button size and window appearance (e.g. rows/columns). In this example the images are (from left to right) 150×150, 200×100, 600×600, 800×400. To my mind the 200×100 looks best:

Same labels for square buttons:

Same labels for supersize buttons: