<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub ChangeWidth(obj As Object, e As EventArgs)
      button1.Width = Unit.Parse("50%")
End Sub
</script>

</head>
<body>

<form runat="server">
<asp:Button id="button1" runat="server" Width="100px" Text="Change" OnClick="ChangeWidth" />
</form>

</body>
</html>