Commuting - How much does bike sizing affect a ride?

To replace my decade old mountain bike, I bought a new hybrid for commuting and leisure. I stand at 6'2 and weigh about 180lbs. I understand the top bar should fall between 3" and 4" from the crotch for a relative fit, but I still have questions about the fit of my bike. I went with a large size frame which I believe is 19".

Mostly, I'm just surprised at how different the ride is from my old mountain bike. Part of this is, of course, because of the different frame design. It's got much larger wheels, so turns are still a little more wide than I'm used to. The thing I notice most is that I ride much higher up. In order to actually mount the bike, I have to sort of tilt it towards me so that my leg can clear the seat and frame. I can take the seat once I step on a pedal in the low position. It just feels very different. Could I be riding too high? At full stride my leg is at a very slight angle (rather than completely straight)

I'm 5'10" and I ride the same size bike. My new hybrid does seem to be higher than than my old mountain bikes. The real question is not height, but whether it's comfortable. I'm not a bike sizing expert, but I do believe your legs are extended properly. You want them to be near fully extended, but you don't want to be stretching, or to be locking your knee. On the seat, with the frame straight (not tilted) you should just be able to touch the ground, on your tip-toes.

It's probably just adjusting to the new position, since you sound like your bike is fitted with similar results to mine, and I made it a point to ensure the guys at my LBS had me fitted properly.

To answer the thread title question, the reason that sizing is important is that it reflects cycling ergonomics. Proper sizing and fitting is done to ensure that you are operating your bicycle in the most efficient body position. This greatly reduces fatigue, soreness, and injury.

It's not bike size so much, as bike fit. You can find a wealth of information about that online. The first thing to check is that seat height is more or less right. Your leg should be almost but not qute straight at the bottom of the pedal stroke. Too high and you rock from side to side on the seat and waste energy and maybe chafe your groin or strain your back. Too low and you can damage your knees.

(EDIT I see you mentioned the leg thing.)

It's normal (or at least common) to have to tip the bike to get your leg over it.

I bought a used bike based on the fact that I could mount it and although I have to stand on the balls of my feet to avoid a lot of pain at intersections the rest of the bike feels good to me. No knee pain when I ride or anything like when I tried the sirrus small. But is there a place where I can go to get the seat and maybe stem ( I don't know if i can adjust that) adjusted for free and without someone trying to sell me a new and much much nicer bike ( that wouldn't be too hard but I love my bike).

I bought a used bike based on the fact that I could mount it and although I have to stand on the balls of my feet to avoid a lot of pain at intersections the rest of the bike feels good to me. No knee pain when I ride or anything like when I tried the sirrus small. But is there a place where I can go to get the seat and maybe stem ( I don't know if i can adjust that) adjusted for free and without someone trying to sell me a new and much much nicer bike ( that wouldn't be too hard but I love my bike).

Is that standing on the balls of the feet while seated, or while standing in front of the seat, over the top tube? If it's over the tube, the bike is too big.

Most bikes aren't designed to let you put your feet comfortably or flat on the ground when seated unless they're specifically designed as "comfort bikes" or "beach cruisers".

You can easily adjust the seat yourself. You either need an adjustable wrench that fits on the outside of the hexagonal bolt at the base of the seat post, or a hexagonal Allen Key, that fits inside a hexagonal hole on the bolt. Or, some seat posts have a quick release lever that you lift and then adjust. The seat height should be set so your leg is not completely straight at the lowest point in the pedal stroke.

Make sure you tighten the seat post bolt firmly when done. Since it's an old bike you can put a tiny scratch on the seat post to remind you where to set the seat height. You also can adjust the seat tilt (it should be level, or a tiny bit higher at the front) and slide it forwards or backwards, and again, either an adjustable wrench or a set of Allen Keys should do the trick. Sometimes you need two wrenches, one for each side.

My top tube is right against my crotch if I'm standing flat on my feet at a stop. The first thing the lady at Performance said was that my bike was too big for me, and that there's no way I could ride it... this despite the fact that I put 6x as many miles on my bike during June as she did. :p

I put thousands of miles on a bike during high school that I couldn't touch the ground at a stop without leaning it over somewhat.

As long as you can stop safely, other measurements of the fit are much more important than the standover height.