14 CFR part 1 defines a small aircraft as an aircraft of 12,500 lbs or less maximum certificated take-off weight. Therefore, any airplane, including transport category airplanes, could be considered small by the Part 1 definition if the airplane is less than 12,500 lbs.

Ultralight airplanes are single-seat, single-engine aircraft that carry up to five gallons of fuel and weigh less than 255 pounds. They can carry only one person, who is responsible for his own safety. Allowed only for recreational purposes, ultralights are flown for sport in areas isolated from busier airspace. Although formal certification is not required to fly these airplanes, training is highly important for safety reasons.

This depends on the plane. Small planes can get anywhere from 500 nautical miles to upwards of 1500 nautical miles depending on fuel load and optional tank configuration. But the middle of that range seems to be the average. I have found that range is more dependent on how long YOU can sit in the plane not now much gas you can carry. Remember there is no bathroom on these little planes and some are not the most comfortable things in the world.