“I would challenge the notion that the Israelites never possessed the entire land God promised them…If we take away the fact that David ruled over the literal promised land, we cut at the root of Messianic prophecy.”

The article is saying that they did indeed attain all of the geographical promised land. Who’s taking away from the fact that David ruled over the literal promised land? Not Dr. Sizer. You have to keep in mind that the Old Covenant “champions of faith” (Heb 11) all looked forward to a heavenly rest.

All these died in faith, without receiving the promises, but having seen them and having welcomed them from a distance, and having confessed that they were strangers and exiles on the earth. For those who say such things make it clear that they are seeking a country of their own. And indeed if they had been thinking of that country from which they went out, they would have had opportunity to return. But as it is, they desire a better country, that is, a heavenly one. Therefore God is not ashamed to be called their God; for He has prepared a city for them.
Hebrews 11:13–16

Be strong and courageous, because you will lead these people to inherit the land I swore to their forefathers to give them. (Joshua 1:6)

The question then arises, did Israel ever do so? While it is true that the Jews have never exercised political sovereignty over all the land between the Nile and the Euphrates, nevertheless Joshua makes clear that in that generation the covenant promise had indeed been fulfilled.

So Joshua took the entire land, just as the LORD had directed Moses, and he gave it as an inheritance to Israel according to their tribal divisions. Then the land had rest from war. (Joshua 11:23)

So the LORD gave Israel all the land he had sworn to give their forefathers, and they took possession of it and settled there. The LORD gave them rest on every side, just as he had sworn to their forefathers. Not one of their enemies withstood them; the LORD handed all their enemies over to them. Not one of all the Lord’s good promises to the house of Israel failed; every one was fulfilled. (Joshua 21:43–45)”

I would challenge the notion that the Israelites never possessed the entire land God promised them. Check out this link: http://www.bible-history.com/map-davids-kingdom/map-davids-kingdom_near_east.html

It’s a map of the kingdom David ruled over. Those boundaries only lasted a short while. But they did exist. I think it is important to recognize this, because as Jews understand it (and I think the messianic prophecies agree) the Messiah is very much a type of David. The whole idea of a Messiah started (I know we can find the scarlet thread throughout the Bible, but the actual word and idea of a redeemer Messiah started) after the first exile.
The Israelites longed to have their kingdom back and began talking about a Son of David who would restore it.
If we take away the fact that David ruled over the literal promised land, we cut at the root of Messianic prophecy.
I don’t think this nullifies Dr. Sizer’s point, and I plan to continue reading, but I wanted to bring that thought up for discussion. Any idea of how we might be able to contact him?