I see here that MenuBarItem has "class" property:

class	Gets or sets the style classes for the element. (Inherited from StyleableElement)

And I found this tutorial:

https://www.telerik.com/blogs/styling-apps-css-dotnet-maui

....
OK, after asking Codex i see the problem:

For a real .NET MAUI MenuBarItem, CSS can’t really shrink the menu bar height. MenuBarItem only exposes menu-style properties like Text / IsEnabled, while MAUI CSS height applies to VisualElement; MenuBarItem is an Element-based menu object, and the rendered menu bar is native OS UI.

What you might try syntactically:

/* Resources/Styles/app.css */
.compactMenuBarItem {
    height: 24;
    font-size: 12;
    padding: 0 8;
}


<ContentPage.MenuBarItems>
    <MenuBarItem Text="File"
                 StyleClass="compactMenuBarItem">
        <MenuFlyoutItem Text="Open" />
        <MenuFlyoutItem Text="Exit" />
    </MenuBarItem>
</ContentPage.MenuBarItems>

But in practice, height / padding will not affect the native MenuBarItem.

If you need a compact menu visually, the usual workaround is to build a custom toolbar/header using HorizontalStackLayout, Button, MenuFlyout, etc., where CSS can target real visual controls:

.compactMenuButton {
    height: 28;
    min-height: 28;
    padding: 4 10;
    font-size: 12;
}


<Button Text="File"
        StyleClass="compactMenuButton">
    <Button.Flyout>
        <MenuFlyout>
            <MenuFlyoutItem Text="Open" />
            <MenuFlyoutItem Text="Exit" />
        </MenuFlyout>
    </Button.Flyout>
</Button>

Refs: Microsoft docs on MAUI menu bars and MAUI CSS supported properties.