Measure the external diameter of the steerer. This should be either 1" (25.4mm) or 1 1/8" (28.6mm).
This is not the sole determinant of star-nut size, as a variety of materials may be used for the steerer tube, each requiring a different wall thickness.
As your internal diameter is at least 24.26mm, this would mean a maximum wall thickness of 0.57mm for a 25.4mm external diameter, which is insufficient even for steel. With a 28.6mm external diameter, the wall thickness would be up to 2.16mm which suggests the steerer is made of some grade of Aluminium alloy.
If you ask your local bike shop for "a star nut for a 1 1/8" alloy steerer", they should be able to supply an item that fits. The star nut will be about 26-27mm in diameter as it needs to embed itself in the steerer wall.
I don't recommend fitting it yourself unless you have the proper tool; It is easy to misalign the nut and bend the tabs.