You can pair as many sets to the one base unit as you wish, even for the whole family. Size fits as normal. Both were amazingly comfortable and Im keeping both in my luggage as vacation only footware. I am a size 10 so ordered a size ten and I am glad i did as they are a good fit. Finally got around to replacing it. My husband and I use our set and I even convinced my sister-in-law to get them and now her and her husband use them. I got them exclusively to wear indoor, but you can wear them outside. Note that Ive never had this problem with the 8 pairs of the Persistent model, which Ive gone back to. I will say I?m not super impressed with the sturdiness and durability. I machine washed the slippers on 30 c degrees, which is the cold setting on most machines, which I google this information before I machine wash the slippers.