Simple Catch theme sidebar

On style.css line 199, this

input[type="text"], input.text, input.title, textarea, select {
 ...
 width: 368px;
}

Is applying a width of 368 pixels to all input fields that allow text entered, hence why your search bar is too wide.

To avoid making changes to original theme files, you should either create a Child Theme or use a Custom CSS Manager plugin. Changes to theme files will erase once the theme updates.

Then within the stylesheet of your Child Theme or the allocated space of the plugin, add this CSS;

.widget input {
 width: auto;
}