command+3 changes to column view, select the pic and that wil show a large preview towards the end, or if you leave it in icon view, select view>show view options> and check show icon prevew to have it show the icon as a thumbnail.
with the window open, click view>show view options, in that window that comes up, there's a slider near the top, you can adjust the icon size, and click all windows or this window only, self explanatory for the most part.
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