First off, whoever told you a Schilke 17 was the same as a Bach 6 is full of it.
Refer to Schilke's web page for a comparison of Schilke numbering to Bach numbering:
Schilke Music Products - Schilke Trumpets and Mouthpieces
Second, Tootsall makes a great suggestion - find (or buy) one of those Warburton mouthpiece kits with like 5 different tops and 4 or 5 backbores and play with them for a couple weeks. It will help you match up your playing needs with the right sized mouthpiece.
Oh, and yes, the Schilke 17 is bigger than a Bach 3, significantly.