Ring sizing stress

I am still having a hard time knowing what size to order for my wedding band, I am eyeing a Tiffany band and unfortunately cannot try on in store because I do not live close by... I have tried on some plain bands in a store one time to get a sense of band width and size, but I am not sure how to pick what size is best as my hands were kind of warm so my fingers were slightly swelled (the Jewelers have just told me whatever size you want, which is not really helpful)... I have to order the Tiffany band online or over the phone and I really just want to choose the right size the first time as I don’t want to stress about sending back and waiting for refund to reorder, etc.... I wish I had started looking earlier but honestly I thought I would have just picked a ring from a local jeweler rather than wanting one from a company that is not close lol .
My engagement ring is a 6 (which is my true ring size), my hands do swell in heat (not a lot but enough) and I know that with two rings on, they feel tighter ... I want to be able to wear the band alone some times so I do not want it to be too big... should I choose the 6 for the band and if need be, resize my engagement ring? I am so flustered that I keep going back and forth, and like I said my local jeweler was no help other than to try to sell me one of his rings lol
(Also the Tiffany band I want is a plain knife edge style/no stones.. could that be resized if necessary even with the shape of it?)
Hoping to figure this out soon, any advice is appreciated..