It's definitely too small. I'm not trying to be rude, but I think the people at the pet store aren't giving you quality info. The only reasons they would be feeding a BP any prey that size is A) cheaper to feed smaller prey and B) they stay smaller longer to look cute and increase sales.
People on here have shared that feeding chart, because it's tried and true. You're not doing your snake any favors feeding prey smaller than they can eat fresh out of the egg. Again, not trying to be a jerk, just trying to help you and your BP.
But, your snake, so do what you feel is best...