I've been fortunately successful in my trading, and I'd like to start upping the quantities of my purchases. Before I might have bought 10 shares of XYZ, but I could stand to buy 100 now. Is there a general rule of thumb, maybe basing it off the spread and/or volume to when you need to start worrying about market impact? I know 100 shares of a security with a packed order book is hardly worth mentioning, but maybe 1000 shares of something that has barely any action might be more of an issue. Just curious if there are any general guidelines or resources out there towards this.