Male to female size conversion for tops & nightgows

[chrisaimes]

 I wear perfect size 8 low rise jeans and size 10 shoes fit like a glove. However, I am having trouble with blouses and other upper body wear. I have some XL tee shirts that fit pretty well. I want to buy a satin nightshirt and don't know what size to get. ......haven't got the courage yet to just go into the ladies dressing room with my nightshirt.  Any suggestions would be greatly appreciated.........