Currently I stacked 3x Mk3 short fuel tanks in it, but a friend did the math and told me it should be able to hold at least 4 of that fuel tank (2,500 units each).

(A 5m diameter sphere is 512 times larger than the 0.625m diameter "Dumpling" spherical tank, which has a fuel capacity of 22 units. This gives it a total fuel capacity of 11,264 units.)

Therefore, this 5 meter diameter sphere, when used as a fuel tank, equals one Mk3 long fuel tank (10,000 units) or ~1.5x S3-7200 fuel tank (7,200 units).

I'm planning to use craft file editing to give a single fuel tank the corresponding capacity, so that it's less part-intensive if I'm to use multiple spherical tanks for my interplanetary vessel.