For me the best way I knew how it fit was how awful another bike felt and how out of control it/I was on basic stopping/starting and shop aisle turns, so when I came to sit on the right one it just felt right.
Things like arm reach to the handle bars can be dealt with afterwards by changing the stem but it will feel awkward, I felt stretched due to not being use to the position for a good couple of rides on it and then I just forgot about it completely.
Also, saddle plays a massive role in comfort. After I bought my first Road bike I thought I'd made a massive mistake on sizing felt great in the shop, but suddenly I was being thrown around and bike rides felt like a rodeo, swapped the saddle and the ride has been beautiful since.