Reduce size of SimpleCaptcha input field

It wasn't setting a size at all so defaults to 20.  It only needs to be
3 characters wide, this sets size to 5.  This fixes the layout of new
Login and Create account forms in Monobook and should be benign
elsewhere.  I29672b63d12cd380d7b00cad3449807da76e4188 made a similar fix
to FancyCaptcha.

Bug: 50387
Change-Id: Ia3098d7c2e28072f52c1cd6615667a96389660dc
Merge "Reduce size of SimpleCaptcha input field"