I've been thinking of buying these Birkenstock ankle strap sandals for a while now https://www.legendfootwear.co.uk/products/birkenstock-milano-bs-birko-flor-unisex-sandals-5825 but I'm not sure if we need to go a size up or down. I have no experience with Birkenstock before. Please if anyone here can help me. My foot size is really small, I'm not sure if they'll fit me