"Why is it small?"
Translation:Proč je to malé?
In your sentences is missing a subject. And that is incorrect.
Note that in declarative sentences "ten, ta, to" can be both - subject - To/ta/ten je malé/á/ý (It/she/he is small) or - demonstrative pronoun -for example: ten kluk je malý (The boy is small.)
But in questions only "to" can be both. "Ta, ten" are only demonstrative pronouns.