how do you change the width of my sidebar?

that would need a good basic understanding of css on your side;
and additional information such as
the name of your theme
and a link to your site.

some themes are easier to change than others.

info also needed:
new width of the sidebar?
change or keep the width of the main area?

Unless you provide a link to your site, no one will be able to help.

sorry.

my site is https://www.red94.net

the theme is vigilance.

i need to be able to fit an ad that is 300×250 in size.

make backup copies before editing any theme files

it might be enough to change the width of #wrapper and #sidebar in your theme options (if there is a option to change css settings) or in style.css of your theme:

#wrapper {width:1030px!important;}
#sidebar {width:300px!important;}

if 300px is too norrow for whatever you need to get into the sidebar, increase it slighly, maybe to 310px;

the !important is to force the width onto the style, as i could not find the style that ultimately defined the widths.

and the banner image https://www.red94.net/wp-content/themes/vigilance_pro/images/banner_v3.jpg needs to be widened from 980px to 1030px.

i should have clarified: rather having everything increase altogether, i would like to have the sidebar wider while the main body is forced to reduce in that corresponding amount of size.

#sidebar {width:300px!important;}
#content{width:650px!important;}

might do.

thank you. i will try this. i greatly appreciate the help.