The size of the Morph Ball has never been explicitly stated. The original description of the Maru Mari described the item as “small and round like a ball” and no game since has ventured beyond this nebulous claim. In fact, the only frame of reference for Morph Ball size is the 1 meter given by scans in the Prime trilogy describing Ball-compatible tunnels, and this, unfortunately, has been misconstrued to mean that the Morph Ball itself has a diameter of 1 meter. This, of course, is ridiculous, as the Morph Ball would have to be less than a meter to boost through a one-meter tunnel with ease.

Ruling this out altogether, there is one tool left to determine the actual diameter of the Morph Ball: pixel ratios. If one fixed height in a two-dimensional game is given, and that object is in the foreground, the approximate height of any other object in the foreground can be ascertained using the ratio of the number of pixels of both objects.

The point of reference for each of the five two-dimensional Metroids is Samus’s height within her Power Suit, 1.9 meters, given by Metroid II’s instruction manual. This can be represented by a well-known sprite common to each game: Samus facing the screen, standing properly straight, but not erect. The Morph Ball, in turn, remains at a constant height within each game itself, so any one of the sprites can be used. With this in mind:

Game Standing Height Morph Ball Height
Metroid 34 Pixels 15 Pixels
Metroid II: The Return of Samus 39 pixels 16 pixels
Super Metroid 48 pixels 16 pixels
Metroid Fusion 40 pixels 16 pixels
Metroid: Zero Mission 39 Pixels 16 Pixels

Another issue arises from this, however: these pixel values are integral, and their corresponding height values could very well be non-integers. A number of pixels equal to x represents a height range from x – 0.5 to x + 0.4999…, or, as we can rewrite the limit for this purpose, x + 0.5 (this is simple: if an object is 10 pixels, it can have a height anywhere from 9.5 to 10.4999… pixels, because there is no such thing as half a pixel: it’s a basic matter of rounding). Now, if x equals Samus’s standing height in pixels, and y equals the Morph Ball height in pixels, the minimum Ball diameter would use the ratio (x + 0.5) : (y – 0.5), and the maximum Ball diameter would use the ratio (x – 0.5) : (y + 0.5). With this in mind:

Morph Ball Diameter Ratio Minimum Maximum
Metroid 34.5 : 14.5 (69 : 29) 33.5 : 15.5 (67 : 31)
Metroid II: The Return of Samus 39.5 : 15.5 (79 : 31) 38.5 : 16.5 (77 : 33)
Super Metroid 48.5 : 15.5 (97 : 31) 47.5 : 16.5 (95 : 33)
Metroid Fusion 40.5 : 15.5 (81 : 31) 39.5 : 16.5 (79 : 33)
Metroid: Zero Mission 39.5 : 15.5 (79 : 31) 38.5 : 16.5 (77 : 33)

The reference point, 1.9 meters, divided by each of the above ratios gives the minimum and maximum diameter values, in meters, for the Morph Ball:

Morph Ball Diameter Ratio Minimum Maximum
Metroid 551/690 m (~79.86 cm) 589/670 m (~87.91 cm)
Metroid II: The Return of Samus 589/790 m (~74.56 cm) 57/70 m (~81.43 cm)
Super Metroid 589/970 m (~60.72 cm) 33/50 m (66 cm)
Metroid Fusion 589/810 m (~72.72 cm) 627/790 m (~79.37 cm)
Metroid: Zero Mission 589/790 m (~74.56 cm) 57/70 m (~81.43 cm)

The absolute range is 589/970 m (~60.72 cm) to 589/670 m (~87.91 cm); unfortunately, within these bounds, the five individual ranges do not cooperate, and only those of Metroid II and Zero Mission coincide. So, to obtain the optimal diameter using this data, there are two options:

1. Take the mean of the five ranges. This produces a diameter of ~75.85 cm.

2. Create an optimized range of values in which the maximum number of games fits. If the diameter is given a range of 589/790 m (~74.56 cm) to 627/790 m (~79.37 cm), three games—Metroid II, Metroid Fusion, and Zero Mission—fit within, while a fourth, Metroid, sits just outside with its minimum of 551/690 m (~79.86 cm). A similar range of 551/690 m (~79.86 cm) to 57/70 m (~81.43 cm)—containing Metroid, Metroid II, and Zero Mission, with Metroid Fusion on the fringe—could be used, but the first is closer to the mean and by consequence closer to the outlier Super Metroid.

Fortunately, both of these options correspond with each other, and though both leave Metroid and Super Metroid on the outside, they appeal to the two most recent Metroid games—Fusion and Zero—in which direction Nintendo will most likely move with its subsequent two-dimensional Metroid features.

In short, the diameter of the Morph Ball is within a range of ~74.56 to ~79.37 cm, and on average is ~75.85 cm.