When creating vector art in Adobe Illustrator or another vector program, the artboard should be sized to fit the icon artwork in order to properly render the previews seen on Iconfinder. If your icon is very small on a large artboard, then the icon preview will be small relative to the image background size. You can leave a few pixels of white space around your icon bounds, but only a few pixels are necessary.
It is also important that the icon artwork is fully contained within the bounds of the artboard. If any part of the icon is outside of the artboard edges, it will be cropped in the icon preview.
Align artboard to pixel grid
For best preview and raster image rendering, it is a best practice to always align the artboards to the pixel grid as seen below.