The viewing angle of the platform distorts the view of the U7 fighters standing on it, but because we know a circle is as long as it is wide, that can be corrected.

From the above you can calculate the viewing angle to be 55.453°

And taking that a step further, you can calculate Goku's true height and the true distance his height represents on the ground:

So the 10px represents 12.141px on the ground. I could have just gone through the simpler route of 448/369= 1.2141 to get the ratio between Goku's apparent height of 10, to the amount of floor that covers, but the above was a more comprehensive proof.

Transferring this to the edge sections of the arena, the hypotenuse accounts for approximately 30% of the arc of one segment.

And the hypotenuse has a measurement of 14.014px, putting the arc at 45.5466px.

From the above, it can be shown that one edge segment represents 1/64th of the circumference of the arena, which would then have a circumference of 2914.9846px and a diameter of 927.8684px.

As calculated above, Goku has a pixel height of 17.634px as well as a true height of 1.75m. This means that the arena has a diameter of 92.082m.

Taking a horizontal line through the centre, the top edge, the bottom edge, then through Goku's feet a proportional and equivalent distance to Goku's position below the centre line. Where Goku's line and the equivalent line cross the red circumference provide points that create single point perspective lines that can then be extended to the left and right edges, joined at the centre line. Every point along these perspective lines is now equivalent to the centre line.

So the centre line, the true width of the arena, is 900px. At Goku's line, the apparent width is 874px. This means that Goku's line is just 97.11% of its true size. This also means that Goku and the edge segment are also 97.11% of their true size too. Thats just the bottom end of the segment though, at the top end, the apparent centreline width is 866px and is 96.22% of it's true size. Taking an average, the segment itself is 96.67% of it's true size.

So the segment nearest Goku is not 45.5466px long, but 47.1172px long, making the circumference of the Arena not 2914.9846px, but 3015.49903px. Goku's height is now not 17.634px, but 18.1586px. For a character of 1.75m height, that means the circumference of the arena is more accurately 290.613m.

When looking at the following, it's worth remembering that because the view is of an angled circle, it's apparent shape is that of being wider at a point below the horizontal diameter that at the diameter itself.

Below, the black dot in the blue circle is both Goku and Vegeta standing back to back, outside the blue circle and inside the red are the Trio de Dangers.

Given that the Trio are very close to Goku's height from this distance, and that it's impossible to separate Goku from Vegeta, I will use one of the Trio for sizing - the smallest of the three dots, since I believe that to be Lavender who is closest in height to Goku.

Using this, it's possible to make a comparison to the size of the block they are standing on. There's not enough info in the frame to get an idea of the angle of view to correct for it, but the size difference is so large it's likely negligible.

The above means that the depth of the segment is 23.43x greater than Goku is tall... approximately. Meanwhile, the radius of the arena is 9.04x larger than the depth of the segment.

Meaning that the radius of the arena is 211.794x greater than the height of Goku... approximately. Knowing that Goku is 1.75m tall, it's possible to approximate that: