Is my bra to small or to big?

my current bra size that i am wearing right now is a 40 B. the Straps always fall down and makes them sag i have to keep on pulling the straps back up all day. i think the band width is to big. i also have it in the last setting and the straps are as small as they can go. the piece in the middle of my bra is not touching my skin is far away so i think the cup size is to small. does anybody know what size i should get? i have tried several sizes and they never fit right. i cant go to a bra fitter because i live in a small town and there aren't any places to go. any suggestions?