speedbar-frame-parameters

speedbar-frame-parameters is a variable defined in `speedbar.el'.
Its value is

((minibuffer)
 (width . 20)
 (border-width . 0)
 (menu-bar-lines . 0)
 (tool-bar-lines . 0)
 (unsplittable . t)
 (left-fringe . 0))

Documentation:
Parameters to use when creating the speedbar frame in Emacs.
Any parameter supported by a frame may be added. The parameter `height'
will be initialized to the height of the frame speedbar is
attached to and added to this list before the new frame is initialized.

You can customize this variable.