Okay this is an incredibly stupid and approximative theory, that I liked just because of how insane it is. I still think I came pretty close though. Enjoy!
The Mata Nui Robot, great receptacle of the matoran universe. In G1, it is told to be 40 million feet tall (or 12 192 Kilometers tall) !
SO, how tall would an official lego set of the Robot be?
To know how tall the set would be, we first have to know the scaling of a bionicle set compared to its real height. It is pretty difficult to know how tall a set would be in real life, but I have found a decent way to find it out!
In this scene of the Mata Nui online Game, we can guess how tall turaga Vakama is. This little statue on the left side of the room is a small easter egg left by Templar Studios is actually from the french speaking comic “Les aventures de Tintin”.
This Arumbaya statue is told, according to the comic and the original real life replica to be about 46,5 centimeters or slightly over 18 inches tall.
Now that we know this statue’s height, we can compare it to Vakama’s. When using photoshop, you can see that the statue comes to about two thirds of Vakama’s height, which means that Vakama is “one third taller” than it.
46,5/3 = 15,5
46,5+15,5= 62 centimeters tall… Vakama is very very small indeed. Not even one meter? Really? That’s pretty disappointing… Good thing he can turn invisible…
Anyway, in real life, the turaga Vakama set is about 8 centimeters tall. A quick equation and we can determine the scaling calculation for the average bionicle set
62 / x = 8 cm -> 62/ 8= X
X= 7,75.
And here you go! In order to obtain the real life height of any Bionicle set (Gen 1 at least), I guess you only have to multiply its real height by 7,75. I recommend using centimeters for more precise measurement 
And now the great awnser: in the lore, The Mata Nui robot is said to be 40 million feet tall, or 12 192 kilometers tall.
12 192 / 7,75 = 1 573, 16 kilometers or 977,52 miles tall !!!
Just to give you an idea of how impossibly huge that is, let me give you some examples!
-The largest bionicle set ever in terms of height is the Kardas Dragon, with 20 Inches (50,8 centimeters). To reach the top of the head of the MNR set, you would need 3 966 774 Kardas Dragons, the equivalent of a small country entirely populated by this one combo-model (for anyone interested, that’s 2 657 738 580 pieces) !!
-The largest tower in existence is the Burj Khalifa, clocking in at 828 meters tall. I would take 1900 Burj Khalifa !!
-The set would be so large that planes would no longer be usable to built the upper part, and astronauts would be required to finish the head !!
But wait, there is more!
Thanks to the MNR being a humanoid robot, we can use the height measurement to calculate its width and length to come up with its piece count and its price!!
Not gonna lie, this is gonna be the most approximate part of the theory. Generally, humans are about:
- 2,7 times taller than they are wide. That way we simply have to divide its height by 2.7:
1 573, 16 / 2,7= 582,65 kilometers wide, from one shoulder to the next.
- 5,1 times taller than they are long. Same here:
1573, 16 / 5,1 = 308,46 kilometers long.
1573, 16 x 308,46 x 582,65 = 282 734 952,4 Km3, which is… pretty reasonable…
But of course, it could not be that easy; as the MNR is not a big rectangle, we have to take into a count its body parts, in particular the head and the arms. We’ll simply calculate all the space that isn’t occupied by the set.
-The arms are both half the size of the whole body, and as wide and long as half the torso area, which leads to a quarter of the whole space being free.
-The head only takes about a quarter of whole shoulder area, and photoshop allows me to calculate that that’ll be about 12% less as well.
-The space between the legs and the slight gap in the abdominal area may leave us to take out another seventh of the whole thing.
-> a quarter of 282 734 952,4 is precisely 70 683 738
-> 12% of 282 734 952,4 is 33 928 194,29
-> a seventh of 282 734 952,4 is 40 390 707, 43
Now for the whole calculation:
282 734 952,4 - 70 683 738 - 33 928 194,29 - 40 390 707, 43
= 137 732 321,7 Kilometers square!!
For this next step, I would like to calculate the piece count. For easier calculation, I will use a new unit: The Kardas Dragon unit, as 670 pieces per 50,8 centimeters (or 0,00508 kilometers).
137 732 321,7 / 0,00508 = 2 711 266 175 Kardas Dragons (at this point we would need a small planet), or
1 816 548 337 000 PIECES!!
A usual good deal for a bionicle set, I feel, is about 17 cents per piece. SO…
308 813 217 400 dollars!!! (Not including the paycheck of all the workers, material, helicopters, rockets and astronauts needed to build it!)
And here you go, we have it all! Hope you enjoyed learning something you never needed to learn 
