Understanding WidthRequest

WidthRequest just describes an element's desired width during the next layout cycle.
For it to work as you'd expect, 2 conditions must be satisfied:
1) the requested width is consistent with all constraits (ex. parent's width) and
2) a layout cycle is triggered.
WidthRequest: https://developer.xamarin.com/api/property/Xamarin.Forms.VisualElement.WidthRequest/

But that's complicated. I'd recommend just replacing the stack layout with a grid, and putting each element in a column of the desired width.
Grid Example: https://developer.xamarin.com/api/type/Xamarin.Forms.Grid/

You need to specify a HorizontalOptions such as "start" or "center". The default horizontalOptions for stackLayout is FillAndExpand, so child elements like a listview will fill the entire available horizontal area even though you specify a width. This was a bad call on behalf of Microsoft because the default behavior will ignore/override a width request.

Here is a visual example: I have a picker where I set the width request to 200, which should take up about 2/3 of the horizontal space.

<StackLayout Padding="10">
    <Picker x:Name="pickerRanks" WidthRequest="200" />
</StackLayout>

As you can see the width request is overridden/ignored. Then if after setting the HorizontalOptions to "Start"...

<StackLayout Padding="10">
    <Picker x:Name="pickerRanks" WidthRequest="200" HorizontalOptions="Start"/>
</StackLayout>

The width request is honored. Of course I'm setting the properties in the .xaml file here, which I usually prefer but you can also set the HorizontalOptions in C# like this

pickerRanks.HorizontalOptions = LayoutOptions.Start;