Thanks for your message.
SharedSizeGroup is a feature exclusive to WPF (not present in Silverlight nor UWP). We plan to implement it as part as the wider support for WPF-only features, expected in late 2019-2020.
A workaround is to use C# to set the column and row width/height to be the same size (feel free to post your XAML code for a more detailed workaround).
To accelerate the development of this feature:
- Please vote for it at: https://cshtml5.uservoice.com/forums/274799-c-xaml-for-html5-feature-requests/suggestions/36585136-definitionbase-sharedsizegroup
- Please contact firstname.lastname@example.org
for a quotation if you are open to contribute financially to ensure that this feature is developed in the coming weeks, with a precise delivery date