the terminology "spatulate teeth" is a reference for crowns broadened above their basal constriction? because in sauropods the spoon-shaped teeth preserv several features typical of the lineage (e.g., lingual concavities, lingual consipcuous medial crest, longitudinal grooves along their carinae, enamel wrinlking), that could help to your diagnosis.

In addition, when you recover a certain type of tooth morphology you could use the time record and associated vertebrate assemblage to help you.  As an example, is unlikely that you recovered a sauropod tooth outside the second half of the mesozoic. 

reading your description, a crown broadened against its root is a common feature present in basal dinosauriforms (silesaurids), ornithischians, basal saurpodomorphs (both lineages with leaf-shaped crowns), but also in lots of cinodonts (incisors as you said) and herbivorous crocodyles (e.g., notosuchia). 

cheers!

at what time does this specimen pertains, and what size does it has?

Sauropod teeth are pretty distinct, both Camarasaurus and Diplodocus are easy to identify and separate from other vertebrates found in the Mesozoic in the United States. Many mammals have spatulate incisors, but are often smaller. Highly worn canines might look like small Diplodocus teeth, especially if they are worn down alot loosing the sharp edge, but this is rare and not likely. Perhaps the most similar would be incisors of tillodonts in the Paleocene, and maybe Eocene taeniodonts, but taeniodonts have distinct grooves along their large incisors. Both groups however don't have that "pencil" shape that is characteristic of Diplodocus and Apatosaurus teeth.