Let's set the scene.
Moses and the Israelites near the end of their journey through the desert. They're pretty much out of the desert proper and finally encountering tribes, kingdoms, and the brutal armies that go with them. They are now east of the Jordan river, heading northwest and headlong into a number of pagan kingdoms.
The first two kingdoms they must pass through are run by King Sichon, ruler of the Amorites, and King Og, ruler of the Bashan area. And right off the bat, the Israelites ask Sichon for permission to pass through his jurisdiction, promising not to go off the highway or attack anyone or steal anything.
Sichon's not buying it.
So, why was Moses afraid of Og? Well, let's hear Moses' version of what happened.
Again, if a cubit were 1 1/2 feet, then Og's iron bed was 13 1/2 feet x 6 feet. He's a giant! Of course Moses would fear him!
Unless of course...we have proof that contradicts this notion.
Detour: let's go for a moment to the construction and dedication of the Tabernacle in the desert. The Tabernacle was like a huge rectangular tent, like a sukkah but with a cloth covering on top. It housed the menorah, incense altar, some other holy relics, and the Ark of the Covenant in the back room.
First, we read descriptions of some of the materials used for the frame of the tent, again using the formula 1 cubit = 1 1/2 feet.
Doing the math, that means each plank was 15 feet tall, not to mention just over 2 feet wide.
But wait...there's more.
So the tapestry covering the Tabernacle was made of 2 gigantic pieces of cloth. Each was 28 cubits long and 20 cubits wide. That's 42 feet long by 30 feet wide...which were laid out on top of the structure, then hooked together up the middle by the loops sewn into them.
Now we read the story of the day the Tabernacle was dedicated...
So Moses set the whole thing up himself. But how could one person erect multiple planks 15 feet high, then throw a 30 foot long tapestry over the top? How else...
So we have an argument that, even if Og were a giant, Moses was pretty big himself. So then why did G-d tell Moses not to fear Og? If Moses wasn't afraid of Og's size, then there must've been something else.
For the answer, we take another DETOUR:
Here's a story from the time of Abraham known as the Wars of the Kings. In my experience, it's another story that most teachers gloss over and don't bother teaching, but for us, it's the key to unlocking the mystery of Og's power.
If Og was the last of the Rephaim, and this is the story wherein the Rephaim were wiped out, then this story would be telling us how Og escaped that fate. Therefore, we equate the unnamed "fugitive" with Og.
So why was Moses afraid to face Og if height wasn't an issue? It was because Og had been protected by G-d throughout his life as thanks for saving Lot, Abram's nephew.
Actually, Rashi was quoting the Talmud below.
Now to be honest, Rashi says it's possible that, even if Og were the one to have saved Lot, his motive wasn't to get anyone freed but, rather, to trick Abram into going to a battle he was sure to lose, after which Og, now without a kingdom and the last of his kind, would marry Sarah and take over their household as the start of a new nation.
So now, back to our story. Og ran out to attack the Israelites, a final affront to G-d, who told Moses not to fear: Og will be delivered to them just like his cousin, Sichon. So here's what happened next.
And with that note that Moses was 15 feet tall, Rashi mentions that we already knew Moses was tall because the Torah stated that he himself, he alone, erected the Tabernacle, which would've required Moses to be incredibly tall to do so. And so, we come full circle, and the opening legend now appears completely relevant to this story after all.