I worked at Subway for years and alot of times I would cut bread while looking up and asking "What type of meat or cheese" and unintentionally cut the bread small. I always tried to put the too small ends in the bottom of the bread cabinet on the "reject" pan, and then waste them at the end of the night.
I would say something, albeit nicely. Just say "I'm sorry, can I get a different piece of bread, that one's a little small" I don't see how they could say No, unless it's the last piece of that type of bread ready in the whole store. They might roll their eyes becasue you're being picky but who cares??

Get your money's worth!!!
I used to have a lady who would come in and order white bread every day, then change it to wheat, then to Honey Oat. Every day. And every day she wanted me to pull out every piece of bread that was ready so she could "see it" and pick the best. She liked it very dark and usually grumbled a little that it wasn't done enough, so I started putting one piece of Honey Oat bread in a pan and almost burning it for her. And every day she was surprised that there was a dark piece just like she liked it
Trust me, asking for a different piece of bread is no big deal. I've had people ask to have a 6" cut into 6 pieces, 1/2 tomato on each section, 1 cucumber, 2 olives, 1 onion, 1 bell pepper slice and 1 dot of mayo on each section, wrapped seperately, so she didn't get her all black buisness suits messy.
You're not THAT picky!!!!