KIK: My largest position is 3% at market value (cost is almost nil). Usually I would limit myself to 1-2%.
As for a 15% position I guess that would depend on your objectives, age, total net worth, other sources of income. Can someone tolerate waking up some morning and seeing 15% of equity wiped out by SEC stopping trading or some other scam. Not for me anymore.
When I started out in the brokerage business I had 1 position which was 150% of my net worth. It worked out well, but since then I have scaled back, way back!
Signatures are so yesterday!