"Αυτός είναι μικρός."

[JacobPast177]

How about "He is young"?

[YPSILONZ]

Yes, absolutely.

[ShaoxuanLi]

this is small should also be accepted.

[fazulakis]

Yes, I thought so too. Unless some Greek speaker enlightens us about some subtlety here.

[Jon345104]

I think you are right

[G.Georgopoulos]

This is small= Αυτό είναι μικρό ;)