Would you never use an article with this type of sentence? In other words, can τι mean 'what a' in a non-interrogative way without an ένας/ένα/μια?
You mean if "Τι ένα ωραίο φόρεμα" would be correct? No, these type of sentences never have an indefinite article. Τι όμορφο σπίτι=what a pretty house, τι ψηλό κτίριο=what a high building etc
Yes, that's right. Many thanks for confirming Troll.
Like in Spanish: "Qué & Que"... ¡Qué bonito vestido!
Why not ωραία φόρεμα?
Because φόρεμα -- like just about all nouns in -μα -- is neuter, not feminine.
Thus it has to be ωραίο φόρεμα.