This will effect anything you put in the position... so I'm sort of assuming here that you're only going to put the Content Particle in the Mainbar Section...
you can use this custom css:
#g-mainbar {
height: 100%;
}
There are several ways you can add custom CSS into Gantry 5:
On the Base Outline or Per Outline Level:
Page Settings > Assets
Page Settings > Atoms
docs.gantry.org/gantry5/configure/page-settings#assets
docs.gantry.org/gantry5/configure/page-settings#custom-css-js
or at a global custom file level
[template-name]/custom/scss/custom.scss
docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
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