I am extruding (Shift+E) a bone with x-mirror on.
The left bone is fine but the mirrored one is HUGE! I have no idea why this is happening. I have a pic and my .blend file is included here.
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Thanks for providing the .blend file, it looks like the issue is due to the Armature being off-center. You can inspect this with the Item tab in the N menu, you generally want an armature's origin to be right at the world center for X-mirror to work.
I was able to correct the issue by translating the Armature object up on the Z axis until the feet touched the floor, and then applying the Location transform to the armature with Ctrl + A. After this, the bones in the feet extrude over X mirror as expected. This also affected the eyes for the character, so you may need to correct their location and scale after applying on the Armature.
You can also apply the 10x Scale if you like, it's a good idea to finalize that early on as it affects poses and animations, but the correct Origin via Location transform is all that's needed to fix the X-mirror.