At standard temperature and pressure (STP) it looks like CO2 has a density of 1.96 kg/m^3. 1 tonne = 1000 kg, so a tonne of CO2 has a volume of (1000 kg)/(1.96 kg/m^3) = 510 m^3 at STP. A cube of that volume would have side length (510 m3)(1/3) = 7.99 m, so roughly 8 meters per side.
I don’t know how tall that person is, but if we assume around 1.6 m (5’ 3") then the cube side length should be about 5 of her. Seems pretty accurate to me.
You had it right the first time, 1 tonne (1000 kg) of CO2 at standard temperature and pressure would have a volume equivalent to that cube.