Size of the creature that burst from the Engineer. . .

I wondered this myself. The normal chestbursters we saw in the Alien movies which came from humans, were much smaller in size but eventually matured into 8 ft. hulking monsters. At first my reaction to the Deacon's birth was looking at how small it was in comparison to a full grown xenomorph. Then I when I thought about it some more, this creature hasn't even matured yet. I imagine the Deacon at full size is gigantic maybe even dwarf the size of the Queen alien. I hope we get to see what becomes of that creature in the sequel if they do make one. Now that would be some spectacle!

Who knows, maybe it wont grow at all? The typical face huggers are fully grown when they're born, but cuddles started tiny and then grew to be huge. Maybe it's an inverse relationship with these huggers/xenos, where the hugger grows, but not the xeno compared tohow the normal huggers don't grow but the xeno does.

Yes, the Deacon, fully grown, "ought" to be around 10 feet tall. However, remember that the Deacon is not EXACTLY the same Xeno that we're used to. It is a "reversed-engineered-bio-engineered-bio-weapon" based off of the original xeno. There is not telling what tinkering has been done by the engineers after they literally sent "all their [xeno] eggs in one basket" (the brother modeled derelict juggernaut now sitting on LV-426 putting out a distress/warning signal waiting for the unweary Nostromo to pass by which was actually sent in that direction by the company a good many months beforehand). They ARE making a sequel (if not two). The first one is tentatively titled "Prometheus: Paradise". It will be an really downer Act II of the trilogy. Expect something like The Empire Strikes Back, but in the "Alien Universe".

Yes we do not know what those movies would bring..... Maybe the end of number 2 sees Engineers set off to LV 223 to find out what happened on that moon/planetiod.... they then come across fully grown Decon and what ever it spawns..