If you are buying shoes/slippers/sandals as/for an obese or very overweight person, then as well as getting a large enough size in length and width, I suggest that you try to get them deeper as well, with plenty of room for the toes. People who are overweight usually have swollen feet - often painfully swollen - and if the footwear is not sufficiently roomy then the pain will be made worse. Similarly with socks - extra roomy is best. And also, with socks, they are more comfortable if they are seamless and if they have a soft top which does not restrict the circulation. Other considerations are whether the footwear is too heavy and tiring to wear, and whether soft insoles would make walking easier.
How to lose weight.