Final answer:
The detection of exoplanets smaller or equal to Earth's size is true; the Kepler Space Telescope has discovered a variety of such planets. Despite initial biases towards larger gaseous planets, it's known now that smaller, terrestrial-like planets are quite common in our galaxy.
Step-by-step explanation:
The statement that at least one exoplanet with a size equal to or smaller than Earth has been detected is true. The Kepler Space Telescope has been extremely effective in finding a wide range of exoplanets, including those that are Earth-sized or even smaller. These discoveries have shown that small, rocky planets, similar to the terrestrial planets in our solar system, are actually more common than giant planets. The challenge in finding Earth-sized or smaller exoplanets lies in the observational bias — smaller planets are harder to detect, which is why there seemed to be a shortage. However, the recent exoplanet detections have corrected for such biases, and it's now known that these smaller planets are not unique in the universe.
The discovery of exoplanets has revised our understanding of planetary systems, with Kepler's data suggesting that about one quarter of stars possess exoplanet systems. This implies that there could be at least 50 billion planets in our Milky Way Galaxy alone, with a significant number of those potentially being Earth-sized or smaller. These planets are part of a diverse spectrum, ranging from "super Earths" which are more massive than Earth, to planets that are less massive and potentially rocky like our own.