Parts taken from articles in exposed on Trumpets site.

Trumpet Bore Sizes

Typical current bore sizes are .450 small, .460 medium, .468 large and .472 XL.
People have stated that horns can't backup, big bores can't play softly......
I can play soft while using a .472 bore trumpet. I have played trombone and can play that softly as well. I have even heard TUBA players play a full sound while playing softly. The bore of a tuba is huge compared to a trumpet. If you have any size brass instrument and sound poorly while playing softly then you NEED to use a closer lip setting. That is a good indication of a spread setting.
When we talk about the air backing up it does not backup in the sense that it travels back to the mouth. The vibrations meet with an increased resistance. I CAN FEEL THIS.
This affects how the lips vibrate and in many cases a less experienced player will end up letting the throat compensate by closing some. If this happens the player has options like opening the embouchure and using less lip curl.
I find that this depends more on the leadpipe than the bore size. I have two 40+ year old Reynolds. One is a .460 and the other is a .468. They are both clean and true to the measured sizes. The bigger bore is a little more free blowing at the low end but since he used a tighter leadpipe on the bigger horn it does backup in the upper register. (If the player does not adjust for it.)
My pic is a .420 and my largest natural horn is a .500. I play them both. I have 20+ instruments inbetween those 2 sizes. I can play all of them also. I can even use the same mouthpiece on all of them. I make all adjustments by the amount of lip curl I use and adjust the sound with my aperture shape. It takes all of 4 - 5 notes on any given horn to do that. In other words I adjust the chops one time for the horn then I don't have to think about it again.
That means I am NOT equipment dependent. I can play any horn or mouthpiece. Now I do have a preference. I like to use lip curl and create my own resistance. So I would rather play open horns that allow me to do that WITHOUT having to adjust.
There are people who make these adjustments by using a combination of air usage / tongue level adjustments. I don't do it that way because in my case there is a difference in sound quality when I rely on the tongue arch.
I mentioned all of that to specifically warn some of you that based on your posts you are TOO DEPENDENT on your equipment. You SHOULD control the horn. It shouldn't control you.
Finally it has been said hundreds of times but I'm saying it again. 'NEVER buy a trumpet that you didn't play first.' And NEVER judge it by the bore size. Bore size is the least important aspect of a horn. Buying a trumpet just because of bore size with out playing it; would be like buying a used car because it had a 454 engine and not checking to see if it started or drove well.

The Trumpet and Its Bore Size - How Critical is It?

There are many choices when faced with buying a professional quality trumpet. One of the first decisions most players encounter is which bore size should I choose? Many players assume that a larger or smaller bore size will create the type of blow they need (i.e. more open or a more resistant air stream). This article will help you understand the concept of bore and help you realize its importance when choosing a new trumpet.
First, the design of the bore is not the size of the hole in the piston but rather it is determined by the size of the inside slide tubes of each of the valve slides (1st, 2nd and 3rd valve slides) and the tuning slide. The leadpipe and bell are conical. The general bore sizes offered on Bb trumpets range from .459″ to .468″.
I would like to first say that the differences in bell size and leadpipe design will change the resistance of the instrument, tonal production and rate of air flow much more significantly than the overall bore size. For these reasons, you should not concern yourself with bore size more than understanding the different leadpipes and bells for each trumpet you sample. Many professional trumpets manufacturers describe the shape of the bell as well as the leadpipe in detail along with giving a bore size. The significance of leadpipe design and bell design has a larger overall effect on the quality of sound and blow than bore size. With that in mind, when looking for your new trumpet, pay attention to the detailed descriptions of the bell and leadpipe and how they effect your playing and overall concept of sound you are trying to achieve.
Generally, the rate of taper in a leadpipe defines the quality of blow (open or resistant). A slower taper creates a more stable, resistant air stream and a fast taper creates an open free blowing air stream. As for bell design, larger bells produce a dark, free blowing feel and medium large bells offer a more controlled, brilliant tone. For example, if your concept of sound involves a darker tone, it would be best to begin with a trumpet that offers a larger bell in conjunction with a slower tapered leadpipe to help balance air flow. My experience has lead me to believe that the best playing instruments are balanced within themselves. Large bells with medium leadpipes or large leadpipes with medium large bells. This balance usually creates the optimum air flow you are looking for.
Where does bore fit into all of this? Basically, it puts it last on the priority list. Balance is what you are looking for. A balanced trumpet will offer a higher quality of sound with more stability and endurance for your embouchure. The ease of playability will encourage your development and ultimately produce higher quality performances.

Article taken from a known link only as order information. My thanks to the responsibles by that.