Well a deli cup is probably about 5" across, maybe a bit bigger, depending on how much potato salad you order
deli cups are probably easier to measure in ounces, I think 5 ounces is standard for the small ones, also a sandwich container is another good container.
rubber maid shoe box is big enough for a pair of shoes. Dimensions are about 4.71 x 8.388 x 13.328 in or 11.96 x 21.31 x 33.85 cm
Sweater boxes have enough room for a couple sweaters... dimensions are about 5.853 x 16.825 x 23 in or 14.87 x 42.74 x 58.42 cm