There's no reliable and generic method for converting specific 4e distances to real-life units. If the fluff for the monster mentions it, go with that. Otherwise, based on a lot of correlating of fluff with mechanics in the Monster Manuals and adventures, I'd estimate a given creature's longest dimension as being within 2 or 3 feet of: the number of squares it occupies in that direction multiplied by 5 (result is in feet, see below for D&D's metric conversions). If the dimension is height, probably add 2 or 3 feet. If it's length/breadth, probably subtract.

That would put a Huge pack lord at maybe 13 feet long (or 18 feet tall, if you think its tentacles at rest reach up further than its horizontal body length--which seems likely given the picture, but I prefer to imagine the tentacles are folded up when not in use), give or take. Its other dimensions can be inferred based on the proportions shown in its art (4e is great about giving us art of most monsters).

A single square on the tactical map is roughly 5 feet (or one meter) to a side.

A 1-inch square on the battle grid represents a 5-foot square in the game world. So a dungeon room that is 40 feet by 50 feet would be 8 squares by 10 squares, which is a huge room but a good size for a busy combat encounter. [PHB1 266, sidebar Visualizing the Action]

(Depending on the country of publication, D&D has a history of converting its standard five-foot grid unit to one meter or 1.5 meters. The latter is more accurate.)

Although there's no official conversion to creature length/height like 3.5 has, the 5x5(x5)-foot square should give you a rough sense of a creature's dimensions...

...but 4e doesn't really care, and it shows.

There are two important things to remember when converting squares to "real-life" measurements. Both arise from the fact that 4e's primary purpose is to be an awesome tactical combat simulator--and it's willing to sacrifice realism and simulation for that goal whenever necessary.

First: obviously a human isn't five feet wide. That width represents the space a person controls in combat, rather than his own physical dimensions. A Huge-size pack lord isn't cube-shaped so much as it's free to wheel and turn within the squares it controls.

Second: most humans are more than five feet tall, but the 4e grid square which a human occupies is a 5-foot cube. That's just... ignored.

So it's nigh impossible to actually say what a displacer beast's dimensions are in metric or imperial units because the squares it occupies are only rough approximations of the space it controls in combat.

And things get even weirder when you measure stuff that occupies multiple squares at once.

Tactical movement on 4e's grid uses taxicab geometry, or Chebyshev distance, so the effect is... often nonintuitive. I'm pretty sure 4e floors are constantly shifting hyperplanes.

I've got no idea how to do a literal unit conversion from Chebyshev distances to "real-life" units for a displacer beast who occupies 9 squares at once, and I don't think it's desirable. In the end, the implications of 4e's tactical movement rules on the physics of the world are clearly unintended and entirely ignored by its creators, and I fully endorse taking the cue from them.

In the PHB we have the following rules concerning a size difference of two or more:

• You cannot move through a hostile creature's space unless it is two or more sizes bigger than you. (PHB 191, Player's Basic Rules 71)
• You can drag or carry a grappled creature but your speed is halved unless the creature is two or more sizes smaller than you. (PHB 195)

But no automatic advantage or disadvantage based on size.

NB Small creatures do attack at disadvantage when wielding heavy weapons (PHB 147, Basic Rules 46), but that is because of their size and the weapon's property, not the relative size of the opponent.