I'm not sure what the dimension of it where or really much else than the story and it's purpose. So my question is if you were to put an elite force within the horse how many could have fit? And on top of that is it realistic that they could infiltrate and open the city gates?
In the first place, this question is not about a historical event, but about a purely mythical narrative: it's no more historical than asking how big Sauron's army is in The Lord of the Rings.
However, on the understanding that finding an answer to the question within the mythical setting of the Trojan War requires detailed investigation of ancient texts, which is to some extent the province of history, the answer will vary depending on which of the following fictional sources you want to rely on.
The Odyssey (7th cent. BCE) doesn't contain a full list of names, but does report that Odysseus, Diomedes, Menelaos, Antiklos, and Neoptolemos were among the Greeks in the horse.
Arctinus' Iliou persis (7th cent. BCE?) doesn't survive, but we do have a report that in his account it was 100 feet long by 50 feet wide (an alternative report, not explicitly linked to Arctinus, makes it 120 feet long by 30 feet wide).
Likewise Lesches' Little Iliad (7th/6th cent. BCE?) doesn't survive, but one report tells us that he had 13 men in the horse. (The manuscripts actually read "3000", but it's hard to believe that that is an accurate report; modern editions usually take it that a scribe at some point miscopied ΙΓ "13" as ˌΓ "3000".)
ps.-Apollodorus Library epit. 5 (1st cent. BCE/CE) reports that Odysseus persuaded 50 men to go into the horse, presumably for a total of 51.
Vergil Aeneid 2 (1st cent. BCE) and ps.-Hyginus Fabula 108 (2nd cent. CE?) tell us that there were nine men in the horse, though their lists differ slightly (both of them name Thessander, Sthenelus, Ulysses, Acamas, Thoas, Neoptolemus, Machaon, and Menelaus; to these Vergil adds Epeus, ps.-Hyginus adds Diomedes).
Quintus of Smyrna Posthomerica 12.314-35 (4th cent. CE?) lists thirty names explicitly, plus "all the other outstanding champions... as many as that polished horse could hold". (The names are: Neoptolemos, Menelaos, Odysseus, Sthenelos, Diomedes, Philoktetes, Antiklos, Menestheus, Thoas, Polypoites, the lesser Aias, Eurypylos, Thrasymedes, Meriones, Idomeneus, Podaleirios, Eurymachos, Teukros, Ialmenos, Thalpios, Amphimachos, Leonteus, Eumelos, Euryalos, Demophoon, Amphilochos, Agapenor, Akamas, Meges, Epeios).
Ioannes Tzetzes Posthomerica 641-50 (12th cent. CE) tells us that there were exactly twenty-three men in the horse, and names them (Neoptolemos, Diomedes, Kyanippos, Idomeneus, Menelaos, the lesser Aias, Kalchas, Teukros, Thrasymedes, Eumelos, Leonteus, Eurypylos, Demophon, Akamas, Antiklos, Peneleos, Meges, Antiphates, Iphidamas, Eurydamas, Amphidamas, Epeios, and Odysseus). There are indications that he's borrowing bits from both the Odyssey and Quintus of Smyrna.