SpecialMaintenanceShell: Fix size of <select> form field

This fixes the size of the script box, which became a multiselect
element of 45 lines high (unlike input fields, where 'size' is
applied to the width).

The problem started in MediaWiki 1.24, in which HTMLSelectField
was given support for the 'size' attribute. Before that the class
ignored this attribute.

Bug: T71075
Change-Id: I240ee969fa7b4582644a9144052b55ff621e90cf