Learn to anticipate your slippage and define your risk. When everyone is leaning on your orders and front running you and you can't get a fill to get out, it's too much size.
If you're asking this question, you probably haven't reached it. Stocks that trade 10 mill + can usually be easy to get in/out of with 5k+ but it really depends how thick the market is. The only way to know is through experience, you have to get burned.
To keep it simple though: too much size is if you miss your out and the slippage is so absurd that you give back the profits on the last 3 or 10 trades.
Also, too much size is if an exogenous event, like a move of several points against you from a market news program, will deplete your financial or mental capital.