These settings are applied to a whole zone, not a block, as far as I can tell.
In any event, I've added an ID called headline to the Block HTML Id field in the zone that contains a Joomla article block.
I've then added a new class called headlinetitle to the div that contains the title tag in the custom block view, which now looks like so:
<div class="headlinetitle"><a href="{link}">{title}</a></div>
Afterwards, I created the following custom CSS rule in the Settings tab > Add custom style sheet button:
#headline .headlinetitle a
{
font-size: 10px;
}
I see that the general settings defined in Settings > Design are still applied, instead the custom CSS rule.