PS6 help in resizing an image please

Morning All,

I'm reasonable in Photoshop, but I cant resolve this little issue, so need some help please

I'm trying to resize an image for a friends web site as a header image, I'm stuck with the constraints of 600 pixels x 200 pixels max The image I'm using is 150mb in size, when I scale it down to the max header size its pixelated when displayed on the page because the file size drops to 200kb approx.

The Question is how do I increase the file size so the image isn't pixelated but meet the constraints of the size please ?

Thanks for the answer John, worked a treat