ProcessStartInfo psi = new ProcessStartInfo();
psi.FileName = "excel.exe";
psi.Arguments = "myfile.xlsx";
psi.WindowStyle = ProcessWindowStyle.Normal;
Process.Start(psi);
should do it, but Google seems to tell me there are some sizing problems with Excel...
ADDED:
On my Win10 PC, Excel365 always opens a file the same way the last Excel session was terminated (on whatever file), so if you close Excel when its document was maximized, the next document you open with Excel will be shown maximized no matter what.