@bradley1982 from the module readme
Styling
This module is set to be 320px wide by default. If you wish to override it, you can add the following to your custom.css file:
.MMM-OpenWeatherForecast .module-content {
width: 500px; /* adjust this as desired */
}
Most important elements of this module have one or more class names applied. Examine the MMM-OpenWeatherForecast.css, mmm-openweather-forecast.njk, or inspect elements directly with your browser of choice to determine what class you would like to override (Pro tip: If you start MagicMirror with npm start dev you'll get Chrome dev tools that will allow you to directly inspect any HTML element in the module).