This works pretty well for me. It also caps the cell width by 90% of screen width.
using HypertextLiteral
@bind screenWidth @htl("""
<div>
<script>
var div = currentScript.parentElement
div.value = screen.width
</script>
</div>
""")
begin
cellWidth= min(1000, screenWidth*0.9)
@htl("""
<style>
pluto-notebook {
margin: auto;
width: $(cellWidth)px;
}
</style>
""")
end