Hi! My two cents... you should get the larger size. I had the same issue. I purchased my regular Chanel shoe size, and the straps were a bit loose. I tried one size down and toe box was a bit tight. I went with my regular size and the straps, while loose, are fine. If it really bothers you and the strap is too loose, get a cobbler to shorten the straps a tiny bit. But wear them a bit first... I love mine and I can just slip them out easily exactly because the strap is loose. Plus, I would not want a stretched out toe box, and now the shoe looks distorted with a "wide fat" toe. The toe is the most important look of a shoe to me. This slingback is very slimming on the feet because of how the toebox.
My friend, who decided to get the smaller size regrets it. Her SA told her it would stretc which it didn't much. It squishes her toes and did not stretch, so she barely wears them now.
My slingbacks are super comfy that I can wear them all day long walking in the city. The chunky heel is great for the sidewalk cracks too!
That's just my two cents. Hope that helps.