The Elgato Game Capture HD editor is made up of a large picture of the current frame of video, and a timeline of smaller thumbnail frames from your video.

 
To make the timeline thumbnails bigger or smaller, use your mouse to click and drag the black bar directly above the thumbnails.  That bar displays the current time of your video, in hours, minutes and seconds.

Click and drag upwards to increase the size of the timeline thumbnails.

Click and drag downwards to decrease the size of the timeline thumbnails.