Hi Chuck,
Thanks for reaching out and thank you for using the Monument theme with BoldGrid Post and Page Builder for WordPress!
You should be able to change the width of the sidebar with a bit of Custom CSS. From your WordPress dashboard navigate to Appearance > Customize > Advanced > Custom JS & CSS and place this bit of code into your Custom Them CSS section:
@media only screen and (min-width: 992px) {
.sidebar-nav {
width: 20%;
}
}
You can change the width percentage to manipulate the page header size.
I really hope this helps Chuck, please let us know if you have any other questions for us!