I'm currently using a Yamaha tenor Trombone and am trying to get a bach 5g mouthpiece. I am currently using a 6-1/2 AL mouthpiece. I just went to my local music store to find one. They had plenty of 5g's, but all of the shanks were to big to fit in my trombone. Do you guys know of any bach 5g's that would fit in my trombone?

you might have a small bore trombone and those are large bore mouthpieces. You can find some on the internet (www.wwbw.com Musiciansfriend.com... etc...) or you may have them special order one for you. But have you played on this mouthpiece before?

My main problem is that I don't know how to figure out my shank size so I can compare it online. It says on the cup, "Vincent Bach 6-1/2 AL" but nothing about the shank size. any suggestions?

There are only really 2 shank sizes, large and small. (Unless you play in Europe and have a European shank, which is mostly common on Euphoniums.)

So, if the large shank mouthpiece is too big, you need a small shank.

Trombone mouthpieces only come in two shank sizes. Bach makes a 5G in a small shank as well as a large shank.
http://www.wwbw.com/Bach-Trombone-Mouthpieces-2c-Small-Shank-in-Silver-i44391.music
There is a drop down box to choose your size. No special ordering required.
or you can try here.. they have better descriptions of what the sizes mean.
Since Bach was on strike for so long , you may have to wait a bit unless where you order from has the mpc in stock. You may want to call before you order.
http://www.hickeys.com/pages/tnmpce.htm

I tryed the small shank too and it still was too big. I was thinking that perhaps I was looking at Bass trombone mouthpieces. If so, are the shank sizes bigger on bass trombones?

Bass trombones are simply large bore.

Small shank was too big? What kind of horn exactly is this? Are you positive it was a small bore piece you tried?

Yes. I am using a very basic looking all gold Yamaha trombone. It doesn't say exactly what kind it is. It looks like the trombone on the main page when you go to the t-bone section.