There is no one size fits all for position sizing. Risk tolerance, ability to take risk, goals in investing, and confidence in your analysis all play a part.
When I first started investing I believed in being concentrated and I figured being fully invested would look like 10 positions at 10%. Problem is when you first start investing If you are maxing out IRA contributions you are doubling your account in year 2, 50% year 3, etc... and a position you hold when you make contributions might not always be a buy when you have cash available. I ended up owning 3 stocks up until I had 50k and I would have been comfortable with 1. I'm now fully invested owning 5 names. This works for me because I have a high risk tolerance, high ability to to take risk, and my goals are to outperform the market, which can be done by being different and taking concentrated positions.
That being said what works for me might not work for you or anyone else. Position sizing is more of an artwork than a science IMO. I think the most important thing is that you are comfortable. If your losing sleep over a large position because it is declining than the position is too big.