You can try to create your own
usertstyle.css[/m] in the [m]conf folder. Also see
https://www.dokuwiki.org/devel:css#user_styles.
In that file you can place CSS code to modify the layout of a DokuWiki template without chaning the template itself.
Assuming that you use the standard DokuWiki template you could try the following CSS code:
#dw__search {
width: 50%;
}
Please try it out, I haven't tested it. If you are using a different template then you will need to tell us it's name so that people can help you.