Nano saltwater tanks are regarded as any water volume up to 40 gallons. All nano skimmers are manufactured with the aim of making a minimal impact on the space available in your tank. Depending on your tank, you may be able to site the protein skimmer in a rear compartment. Most nano skimmers are designed to hang on the side of your tank with the skimmer body submerged and the collection cup above the water level.
The addition of a skimmer in a nano tank usually means your tank will need to be open top to allow the placement, unless you add a Biocube Protein Skimmer to a Biocube aquarium. Manufacturers usually include a few hanging clip types suited to different tank top designs.
It is also worth remembering that although most nano skimmers have filter-down technology from larger far more expensive skimmers, they are limited in their ability when compared to their larger counterparts due to the limited footprint of their design. Nano protein skimmers can be a valuable addition to a nano reef tank and the the money and size they do perform admirably, but do not the expect high end performance you would have from a larger more expensive skimmer.