Looking at the default HubTile Control Template, the Notification text is configured like this:
...
<TextBlock x:Name="NotificationBlock" Grid.Row="0"
Margin="8,8,0,6"
Text="{TemplateBinding Notification}"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeLarge}"
Foreground="{TemplateBinding Foreground}"
TextWrapping="NoWrap"
LineStackingStrategy="BlockLineHeight"
LineHeight="32"/>
...
If you want to change the font size, you will need to make a copy of the Control Template and change the relevant properties.
The easiest way to achieve that is to edit the Control Template using Expression Blend.