Here's the code that I use to extract the icon size that I want:
Dim i As Icon = My.Resources.Spectrum
Using i2 As New Icon(i, New Size(256, 256))
Me.PictureBox1.Image = i2.ToBitmap
End Using
This works from 16x16 up to 128x128 but for 256x256 it extracts the 128x128 icon. I tried 0x0, because I seem to remember that that is how the large size is stored in the meta data, but that didn't work either.