Hi,
For simple inductor calculations the length of the magnetic path is of concern. That's the total length the flux would pass through. So for a core with square OR rectangular OR circular cross section (as long as it was uniform) and say it was a rod bent into a square shape with ends touching perfectly with no gap, then the magnetic path would be one side multiplied by 4 (because a square circumference is simply c=4*side). The actual length though is the *mean* length, so if the inside of the core on one side was 2 and the outside of the core on one side was 4, then the mean length of that one side would be 3 (just the average of the two). That would make the total magnetic mean path length equal to 12.
If the core was a circle and the inside diameter was 2 and the outside 4, then the mean diameter would be 3 and so the magnetic path length would be 3*pi.
So the length used for simple inductor calculations is the mean magnetic path length which is the mean magnetic path length of the entire core.
What this means is that if we had core material that was 1 inch square cross section for example and we made a core that had 4 inches mean mag path and one that had 8 inches mean mag path length, the smaller one would have a higher inductance than the second.