Try something like this
Get-Template | Select Name,@{N="Size";E={($_.Extensiondata.layoutEx.File | Measure-Object -Property Size -Sum).Sum}}
The size is expressed in bytes, if you want KB, you can do.
Get-Template | Select Name,@{N="Size";E={($_.Extensiondata.layoutEx.File | Measure-Object -Property Size -Sum).Sum/1KB}}
You can replace the 1KB by 1MB or 1GB to get MB or GB respectively.
If you want the size nicely formatted you can do
Get-Template | Select Name,@{N="Size";E={"{0:f2}" -f (($_.Extensiondata.layoutEx.File | Measure-Object -Property Size -Sum).Sum/1KB)}}
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Blog: LucD notes
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