How do I know what size bike I need? If the LBS isn't all that helpful, and betrayed my trust, what are some rules to go by?

I am slightly over 6'3", and 6'4" with shoes. I tried following Shelden Browns advice when adjusting my seatpost for height, and it is high up there. I am wondering if I bought the wrong size, and which size should I consider on my next bike purchase.

sheldon says that you should have your legs fully extended without wobbling side to side. I almost need a rolling start to mount it when my seatpost is that high.

My LBS sold me a 19 inch frame, then later let me swap it out for a 21" trek MTB

Find another shop

Read this from Sheldon Brown and pay attention to the video clip Starting and Stopping

Did you read this from Brown? Revisionist Theory of Bicycle Sizing

So I have been kicking it cowboy style lol.
I will work on that. First my bike was too small feeling. I can almost flat foot if my seat isn't up very high. I leaned cowboy style when expanding my seat post so my legs can fully stretch out while pedaling.

As to the bike sizing, I understand that bikes have a dishonest or inconsistent sizing techniques. I did read that. Is there a more simple guideline?

I bought my bike from a shop that was recommended, and respected in the area. My sales person must have been slightly green. I visited a second shop in the area, but it recently opened and the guy was young. I went there to look at a brooks saddle which was recommended to me from other forum members and the sales person put down on it. I got this second bike stores location from the Brooks Website. I do not think I'd get proper service, unless an experienced tech or the owner would help me.