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.