You can pick your cup size to match your breast forms or by how much padding you want to put in your bra cups. Almost anything can be used from water filled balloon's to birdseed and sponge rubber. The type with hollow back's mold over the male pectorals and give the best fit. It is also best to buy the next size smaller, because the forms will fit better in your bra. A hairless chest will look and feel better than one with hair, especially if an adhesive is used.
Panty size is determined by hip size.
Slip sizes correspond to bust measurements. Many customer's to Large or Tall girl stores are men who buy for themselves. At least you won't run the risk of getting ripped off by a clerk or company who will sell you anything just to get rid of you. Most store's and mail order companies want to have a satisfied customer, who will buy from them again. Another possibility is making your own clothing or finding a seamstress to make garments from patterns and cloth you purchase.