this is small should also be accepted.
Yes, I thought so too. Unless some Greek speaker enlightens us about some subtlety here.
I think you are right
This is small= Αυτό είναι μικρό ;)
But one could be referring to an thing in the masculine gender!
I thought about this for a minute, you see, the better translation would be "This one/That one is small".
How about "He is young"?