Snippets are 1 or 1.5 seconds long. With a Pro subscription, you can make them up to 10 seconds long, as long as the video you're cutting from is at least that long.
Choose the length from the options on the snippet selector screen. It changes in steps of 0.5 seconds.
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Once you set the length of your snippet, all subsequent snippets added to that project will automatically use the same length until you change it again.