You can change MinWidth and MaxWidth
For example I get a big header by changing "Document 1" header to 500 in a test from WinUI 3 Gallery :
<muxc:TabViewItem Header="Document 1" MinWidth="500" MaxWidth="500">
<muxc:TabViewItem.IconSource>
<muxc:SymbolIconSource Symbol="Placeholder" />
</muxc:TabViewItem.IconSource>
<samplepages:SamplePage2 />
</muxc:TabViewItem>