mailzaforume77 Would suggest to add certain and specific media query css for certain breakpoints and adjust width value accordingly :
@media only screen and (min-width: 320px) and (max-width: 480px){.navbar-form.span-3{width:87% !important;}}
@media only screen and (min-width: 768px) and (max-width: 992px){.navbar-form.span-3{width:87% !important;} }
@media only screen and (min-width: 992px) and (max-width: 1200px){.navbar-form.span-3{width:87% !important;} }
Thereon, You may debug and examine further with assistance of browser inbuilt emulator - firefox and chrome