Size (And more importantly height and mass) matters

As stated. I'm terrible when it comes to properly gauging sizes. As a result when a player asks me how tall a room is, I'll usually state that its about 8 or 9 feet (if I don't want people flying around in it) or some outrageously massive number (if I do).

More to the point though, a part of the 'problem' of size I think arises from the grid that 3e and Pathfinder and the like. On a grid, a large creature is 'only' 10' wide. And a huge is only 15' wide. And so on. A colossal only ends up being a few squares on the table.

Now, we can envision colossal pretty easily, but I think the problem goes lower. It goes to large creatures.

Back in 2e, the size system was more vague. No real statistical import was built in besides weapons doing more damage and the like, but in 2e, size was given in terms of the creature's dimensions. So the concept of a barn sized monster felt more like, well, a barn sized monsters. In 3.5 and up, we started thinking smaller, instead of larger.

Similarly, people started losing track of just what it meant to be large.

A large creature ranged, in theory from about 8 to 14 feet tall. I want to restate that. 14 feet tall.

Giants, who I have heard people comment 'aren't that big' are typically at that top end, with hill giants being about 12 feet tall. That's FREAKING BIG.

12 feet is basically two stories tall, meaning a hill giant is about the height of a suburban house.  That, as I stated is what 'large' means. Huge and the others are commensurately much larger.

An underground ruin that can accommodate a retriever, a dragon or a huge spider, needs to be significantly large, like stupidly massive, to account for that thing living in it.

And that is where problems start to come in for people without good gauges of size. Large underground monsters, slithering creatures and the like, need massive space, need enormous space, and finding that sort of enormous space in a dungeon or a cavern system damages credibility if someone can't think of a good reason for a cathedral sized opening carved in the living rock, or why a dungeon-builder would spend all that money for huge 'wasted' space.

Tribes of giants living in caves, need sodding enormous caves to live in, just by being large because again, the height of a two story building. Huge or God help you, Colossal beings even more so. Now the nature of the beast (oozes, snakes, etc) may make this easier, but it needs to be accounted for.

I find its helpful to think in terms of landmarks. Stuff that you can put into your mind as a marker for things of certain sizes. And remember, even in 3.5 and Pathfinder, Colossal being 5X5 is the minimum for colossal.