There are many sites that purport the Sun's diameter in different measures ranging from under 864,000 miles to as much as 870,000 miles across. The Sun expands and contracts at different stages of its cycles and therefore what can seem to be differences in size. However, the mean diameter is 864,000 miles. If you Google "Sun 864,000 miles" you'll get about 6,000 hits referring to the diameter of the Sun being 864,000 miles. As well as the recognized astronomical authorities such as Astonomy Magazine (see second paragraph under "SIZE") and NASA (located under the second picture of the Sun).

One proof, better than Google, is a Total Solar Eclipse. A Total Solar Eclipse occurs because the Sun and the Moon appear to be exactly the same size in the sky and the Moon covers the Sun precisely. However, in reality, the Moon's diameter (2,160 miles) is 400 times smaller than the Sun, but the Sun is 400 times farther away. Hence the Moon's diameter being 2160 miles appears to be the same size as the Sun (2,160 miles x 400 = 864,000 miles). If those numbers are off by even a small difference there would be no “Total Eclipse” as is seen from Earth.  There would only be a completely blacked out Sun if the Sun was smaller than it is or the Moon bigger than it is and only a Partial Solar Eclipse if the Sun was larger than it is and the Moon smaller than it is.