"hard to hold onto"
I don't buy that. I felt the iPad mini is comfortable to use, I don't think the 6+ should be a big deal at all.
However, opinions can vary, so if you don't like it, I think you can return the device back in over the next 2-4 weeks, depending on contract, product return laws, etc. You might be out some money though, restocking fee might apply, and there is a bogus (IMO) non-refundable sign-up fee, at least for several US carriers.
I think what's really bizarre is to buy an expensive device without trying it first. It's a rather personal device and you're buying it for daily use.