"Αυτός είναι μικρός."

Translation: He is small.

[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 ;)

[deryckchan]

But one could be referring to an thing in the masculine gender!

[G.Georgopoulos]

I thought about this for a minute, you see, the better translation would be "This one/That one is small".

[JacobPast177]

How about "He is young"?

[YPSILONZ]

Yes, absolutely.