How does one know what is the best cassette size for your bike? I've been riding for about 6 months now. I have a 53-39 chainring and 12-25 cassette. It appears the new model of my bike uses 12-27. I mostly ride flats and average around 15-18 mph on my rides (with stops due to traffic). Most of the time ride use the 39 chainring but find that some times have to move up the the 53 chainring (but use a bigger sprocket size) since I'm spinning too much on the 39 chain ring. Would a 11-23 or 12-23 be better? Or should I move to a 12-27 and spend more time on the 53 chainring with a larger sprocket? How does one know what size is right or should I not worry about cassette sizes so much?

How does one know what is the best cassette size for your bike? Would a 11-23 or 12-23 be better? Or should I move to a 12-27 and spend more time on the 53 chainring with a larger sprocket? How does one know what size is right or should I not worry about cassette sizes so much?

If you regularly find yourself spinning out the 53/12, then you should consider a cassette with an 11. Otherwise, why have a gear you'll never use? If you regularly find yourself struggling up hills in the 39/25, then you should consider getting a 27. Otherwise, why get a gear that you don't need? If you find yourself never spinning out the 12 or struggling in the 25, then a 12-23 might be a great cassette for you, since it would have closely spaced gears. All that said, maybe the best thing for you is to just wear out the cassette you have now, and by the time that happens you will probably know whether you need to change gearing. BTW, riding larger cogs with the large chain ring does reduce drive train wear and generates slightly less friction.

...I mostly ride flats and average around 15-18 mph on my rides...

From your description combined with Kerry's analysis the 12-23 sounds like an it would make a good, next purchase. Not that you really need to as it sounds like the 12-25 is working for you. Ignore what they are putting on your stock bike right now as the 12-27 is there to accomodate a wide range of abilities and terrain. If you do decide to buy a 12-23 do not get the Dura Ace as a lower level component works perfectly well and is far more cost effective. Keep the 12-25 if you find yourself taking a vacation with some hills.

Don't worry too much about not using the big ring often as long as you are spinning well and not crosschaining.

...How does one know what size is right or should I not worry about cassette sizes so much?

Keep track of what gears you use and dont use on your current cassette. Look at eliminating what you dont use and filling in the middle. In this case if you are not using the 25t cog then getting the 12-23 adds an 18t cog in the middle which should get a lot more use in your case.

I agree with everyone's advice, nothing wrong here. Sounds like when you wear out this cassette, get a 12-23 for your next. No need to change now, it's not enough of a difference to warrant the expense.

just wear out the cassette you have now, and by the time that happens you will probably know whether you need
+1...

...or should I not worry about cassette sizes so much?

Just ride.