How to increase the size of a SymbolIcon in XAML?

Not sure if this is the best way to do it, but it worked for me and might work for you:

<Button Style="{StaticResource PlayButton}">
    <Viewbox MaxHeight="200" MaxWidth="200">
        <SymbolIcon Symbol="Play"/>                                
    </Viewbox>
</Button>

You could use a TextBlock with FontFamily="Segoe UI Symbol" Text="" and then setting FontSize works. If you look at the Symbol values - you can see the 57602 is the value of the Play symbol enum which corresponds to the character code in "Segoe UI Symbol". More typically these values are written with their hex values as in Text="", but the decimal is easier to find if you look at that enum's documentation.