For me, a full position is about 10% of my account. I only trade in my brokerage and ROTH, but primarily in my brokerage because thats the only place I can get significant funds in and out.
The other day I started with half a position of TGT. If my brokerage account is 200K, I bought 10K worth of TGT.
My thought process is, IF I'm wrong, which is quite often and the stock falls to a better price and I still like it, I will fill in with more shares. IF I'm right, well my porftolio has grown, so I'm still happy.
I only want to track about 10-15 stocks, and I don't want to be too overweight in any stock in particular as I want to be diversified.
Right now, only 2 stocks are over 10% of my portfolio - GOOG and WMT. I have a lot of GOOG. GB GOOG.