What planet is the same size as Earth, has no moons, and has a thick atmosphere of mostly carbon dioxide?

(Assuming you mean within the solar system)

That would be Venus. Venus and Mercury are the only two known planets without moons, and Venus is not only extremely close to Earth in size but is known for its atmosphere, which is so thick heat from the Sun gets trapped in it, creating a greenhouse effect and making Venus the hottest planet in the Solar system despite being farther from the Sun than Mercury is.