Style FontSize OnPlatform in Xaml

Hi everybody, i'm looking for a way to change the fontsize of all my labels with a style defined in App.xaml (XF 1.3).

But i don't find any way to make it works.
Note : i want to make it with xaml in app.xaml

<Style x:Key="Titre"
       TargetType="ctrls:Label">
    <Setter Property="FontAttributes"
            Value="Bold"/>
    <Setter Property="ctrls:Label.FontSize">
        <Setter.Value>
            <OnPlatform x:TypeArguments="x:Double">
                <OnPlatform.iOS>18</OnPlatform.iOS>
                <OnPlatform.Android>18</OnPlatform.Android>
                <OnPlatform.WinPhone>26</OnPlatform.WinPhone>
            </OnPlatform>
        </Setter.Value>
    </Setter>
</Style>

The bold attribute worked but no luck with fontsize.

I've tryed different solutions :
<OnPlatform.iOS>
18</x :Double>
</OnPlatform.iOS>

Even this line doesn't work :
<Setter Property="FontSize" Value="30"/>

I can declare style for everything except fontsize. What am i doing wrong ?

(please Xamarin Team, do something for the code we paste here : there are always some text hidden, some lines missing etc. I don't know how the hell you came up with such a weird behavior but i d rather prefer simple text without formating that half the code i paste. We are not here to edit our message 10 times. I spend more time editing my code than writing the entire message. Thank you guys)

Exemple : my code OnPlatform.iOS x double 18 shows half the code in my browser. So people will think i don't care about this post because there are code missing but no. The text is just hidden for no reason.