The maximum size of the world border is 59,999,968m, and that's also the initial size. However, the initial size used to be 60,000,000 before Minecraft 1.17. When upgrading from an old world, the world border becomes the correct size, but the stored value remains 60,000,000. This means that the /worldborder get command returns the wrong value.

Steps to reproduce

1. Create a world in Minecraft 1.16.
2. Open the world in Minecraft 1.17 or a newer version and run /worldborder get
   • It returns 60,000,000. Expected: 59,999,968.
3. Use a program like NBTExplorer to inspect the level.dat file
   • The Data.BorderSize value is 60,000,000d.
4. Teleport to the edge of the world (/tp @s 29999999 ~ 29999999)
   • Notice that the world border is 59,999,968m wide, not 60 million.
5. Run the command /worldborder set 60000000
   • It gives an error message: "Double must not be more than 5.9999968E7, found 6.0E7"
6. Run the command /worldborder set 59999968
   • It is successful, but nothing changes about the world border.
7. Repeat step 6.
   • It gives an error message: "Nothing changed. The world border is already that size"
8. Repeat steps 2 and 3.
   • It correctly gives 59999968 this time.