Your heel should rest fully on the sole and not hang over the edge.
If you frequently wear thermal or fuzzy socks, experts at Aerothotic suggest ordering a size larger than your usual footwear to prevent your toes from feeling cramped. Material Behavior: what size slippers should i buy
Many slippers are sold in "Alpha" sizes (S, M, L). According to Dearfoams and Kohl's , the standard conversions are: Your heel should rest fully on the sole