You can pinvoke to ShowWindow with SW_SHOWMAXIMIZED to maximize the window.
Pinvoke.net has an entry for ShowWindow here.
For example,
// Pinvoke declaration for ShowWindow
private const int SW_SHOWMAXIMIZED = 3;
[DllImport("user32.dll")]
static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);
// Sample usage
ShowWindow(proc.MainWindowHandle, SW_SHOWMAXIMIZED);
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