if aliens are watching us, from millions of light years away, with their advanced telescope

To work out what minimum size of telescope would be needed, we can apply the Dawes limit.

To detect a large 10m dinosaur from 1 million light-years away with a diffraction-limited optical telescope would require a telescope about 50,000km across.

Lets just ignore clouds, atmospheric distortion and the fact that your average dinosaur was more the size of a chicken than a T-Rex!

(Let alone

*multiple* millions of light years...).

My rough calculations (please check!):

Dinosaur length: 10m

c: 3.00E+08 m/s

1 year: 3155760 seconds

1 light-year: 9.47E+14 m

Angular resolution required for detection: 1.06E-14 radians (small angle approximation)

yellow light: 5.80E-07 m wavelength

Minimum telescope diameter: 5.49E+07m, or 54910 km (from the Dawes Limit)