C# - Working with high resolution screenshots

I need to capture an area within my desktop. But I need this area to be very high resolution (like, at least few thousand's pixels horizontal, same goes for vertical). Is it possible to get a screen capture that has high density of pixels? How can I do this? I tried capturing the screen with some AutoIt script, and got some very good results (images that were 350MB big), now I would like to do the same using C#.

Edit:

I am doing my read/write of a .tif file like that, and it already loses most of the data:

using (Bitmap bitmap = (Bitmap)Image.FromFile(@"ScreenShot.tif")) //this file has 350MB
{
    using (Bitmap newBitmap = new Bitmap(bitmap))
    {
        newBitmap.Save("TESTRES.TIF", ImageFormat.Tiff); //now this file has about 60MB, Why?
    }
}

I am trying to capture my screen like that, but the best I can get from this is few megabytes (nowhere near 350MB):

using (var bmpScreenCapture = new Bitmap(window[2], window[3], PixelFormat.Format32bppArgb))
{
    using (var i = Graphics.FromImage(bmpScreenCapture))
    {
        i.InterpolationMode = InterpolationMode.High;

        i.CopyFromScreen(window[0], window[1], 0, 0, bmpScreenCapture.Size, CopyPixelOperation.SourceCopy);
    }
    bmpScreenCapture.Save("test2.tif", ImageFormat.Tiff);
}

Solution

You can't gather more information than the source has.

This is a basic truth and it does apply here, too. So you can't capture more the your 1920x1080 pixels at their color depth.

OTOH, since you want to feed the captured image into OCR, there a few more things to consider and in fact to do..

OCR is very happy if you help it by optimizing the image. This should involve

reducing colors and adding contrast
enlarging to the recommended dpi resolution
adding even more contrast

Funnily, this will help OCR although the real information cannot increase above the original source. But a good resizing algorithm will add invented data and these often will be just what the OCR software needs.

You should also take care to use a good i.e. non lossy format when you store the image to a file like png or tif and never jpg.

The best way will have to be adjusted by trial and error until the OCR results are good enough.

Hint: Due to font antialiasing most text on screenshots is surrounded by a halo of colorful pixels. Getting rid of it by the reducing or even removing saturation is one way; maybe you want to turn it off in your display properties? (Check out ClearType!)