Menu width

I would like to reduce the width of the menu. where can i set that ?

Its not possible with 2 clicks. You need some css skills to do this.
The "jp-left" where the menu istored has a width of 300px and the main content has a margin-left of 300px.
Please take a look at this sizes at the file > css / joomlaplates.css

#jp-right {
margin-left: 300px;
max-width: 100%;
position:relative;

}

#jp-left {
width: 300px;
float: left;
padding: 0;
position:relative;
}

But please do not edit the org files. If you want to customize anything, put the custom code into the custom.css