I have a new 15.3 hand horse, she has a fat tongue where it bulges out the sides of her teeth so I want to get a low port tongue relief bit. Her lips are kind of fleshy and I'm wondering if she needs a bit with a longer mouthpiece than 5.5 in. Is this common...? I guess I always just assumed that 5 in mouthpieces worked on pretty much all horses that weren't drafts. The chiro I use said she a large mouth and she advised I may need a 5 1/4 or 5 1/2 bit but we didn't measure. I borrowed my friends 5 in low port yesterday and her lips do go all the way to each end of the bit, (is that called the purchase lol?). The only bits I see that offer larger in low ports are mylers and I don't want to spend that money unless I have to. 

Have you looked into english bits? They offer way more sizing options. Depending on the type of bit you are looking for.

yes its possible, i had a big qh gelding that needed a wider bit. the sides of the bit- the purchase- would dig into his jaw/cheeks. Also his mouth was almost being squeezed by the mouthpiece of the bit. So I ordered a 5.5" from Dutton.