Hi Hiroki,
To clarify, what do you mean by “normal conditions”? Are you wanting to modify the width only on desktop?
If so, here is a CSS you may try adding in Appearance > Customize > Additional CSS:
@media (min-width: 1025px) {
form.search-form.navigation-search {
width: 50%;
margin-left: auto;
}
}
Kindly let us know if you’re wanting something else. It might help if you can create a sketch of the look you’re going for.
Hope to hear from you soon. 🙂