Add some pictures. Measure it exactly, can give us a better idea. The detent ball is 6mm. There could have been a spring there too, but not for sure.
It might be a ball from one of the bearings. It would be larger than 6mm, unless it is from the throwout bearing or clutch pushrod bearing.
Fine shavings are okay. Normal on a K: there's no oil circulation or filtration on the transmission. Feel them between your fingers. They are a problem if you can feel chunks.
The concern with continuing to drive like this is a transmission (and the rest of the driveline) lockup. Before you ride it more, you should figure out the problem.
If it's the detent ball only, probably not an issue, though finding neutral could become very hard. The spring may be acting as a detent, but it is not hardened or rounded for this purpose. There is some concern that parts could break off and become jammed. Most likely this would not happen, you'd just end up with more junk on the bottom.
If it's not the detent ball, you could be in for trouble. If it is for one of the major bearings, I would replace the transmission before riding more.
If it is for the throwout bearing, you can actually check and replace that from the rear of the bike, with minimal issue.
