I was experimenting with the new .net maui preview 13 and I created a new app, but for some reason the title bar on windows is looking weird 
As you can see the the title bar is grey (not white) and it is thicker than normal (normally it is as thick as the three buttons) and there is no icon like the other .net app, So is this behavior normal and if so how can I change the title bar color and size?
Note: after some investigation I discovered that this only occurs with the content page
The height or any other size i think it depends on the platform you are running your App. I'm not sure that can be done.
But for colors you can try this. Add to App.xaml:
<Application>
<Application.Resources>
<ResourceDictionary>
<Color x:Key="PrimaryColor">#242A75</Color>
<Color x:Key="SecondaryColor">White</Color>
...
<Style TargetType="NavigationPage">
<Setter Property="BarBackgroundColor" Value="Black"></Setter>
<Setter Property="BarTextColor" Value="{DynamicResource SecondaryColor}" />
</Style>
...
</ResourceDictionary>
</Application.Resources>
</Application>
That will set Titlebar background color to Black, and text to your predefined SecondaryColor defined. This will apply to all the titlebars in your app.