You can adjust the DPI, zoom in, and full screen the Materials panel to make it bigger, then take a screenshot.
Make the Properties editor full screen by hovering over it and pressing Ctrl↑
Increase the DPI in User preferences > System
Zoom in on the panel by holding CtrlMMB and dragging the cursor
Result: (click for full size)

The obvious limitation being that you can't go larger than the maximum zoom, or larger than your monitor.
Get the blend:
Alternatively, you can download the blendfile from the GIT repo with your browser via Phabricator:
To download a file, go to the link above and click Download raw file on the right:

Then click Download:

The different preview scenes are on the following layers:
Layer 1: Flat XY plane
Layer 2: Sphere
Layer 3: Cube
Layer 4: Suzanne (Monkey)
Layer 5: World Sphere
Layer 11: (the first layer on the second row) Hair