OnPlatform XAML

Hi!
I am trying to add the OnPlatform tag in my XAML ContentPage definition file.
I have a working Android App and now wnat to to make tweaks for the IOS version.
I would like to use the OnPlatform tag to detect whether Android or IOS are being used.
I would also like to detect whether a tablet or phone is in use, but I did not see a tag for this.

1) I tried to add the OnPlatform to my buttons to set the font, but then the text on the buttons just disappeared
OnPlatform version:

<Button>
  VerticalOptions="CenterAndExpand" Text="{Binding ItsVal}" Clicked="OnAquaDeviceCellClick"
  <OnPlatform x:TypeArguments="Font"
    iOS     ="15"
    Android ="13"
    WinPhone="14" />
</Button>

Generic version that works:
<Button Font="13" VerticalOptions="CenterAndExpand" Text="{Binding ItsVal}" Clicked="OnAquaDeviceCellClick" />

2) I tried to add the OnPlatform in my ListView to set the RowHeight but got a Java exception on Android
OnPlatform version:

<ListView>
      <OnPlatform x:TypeArguments="RowHeight"
        iOS     ="50"
        Android ="40"
        WinPhone="30" />
        ItemsSource="{Binding}" ItemTapped="OnGetRidOfSelection"
        ItemTemplate="{local:DataTemplateSelector Page={x:Reference thePage}" 
    </ListView>

Generic version that works:

<ListView RowHeight="40" ItemsSource="{Binding}" ItemTapped="OnGetRidOfSelection"
             ItemTemplate="{local:DataTemplateSelector Page={x:Reference thePage}" />

What am I doing wrong?
Do I have to use the code "Device.OS" to do this?

Thanks!