how to change field size in contact form

Hi, can anyone tell me how to change the size of the different fields in a contact form?
The fields in my default form all have the same size.
I already changed some values in the contact.xml file (in the components\com_contact\models\forms folder) , but nothing happens.
Does anyone have any idea?
The template i used is exchange for joomla 3.8.
Thanks

It is based on basic Joomla / Bootstrap css styles.
You can change it with your own css/custom.css

Find the css and create an override
www.joomlaplates.com/template-customizing