How far can I safely size up or down

[Mjay]

I need advice. I just purchased a 5 stone ring and had the seller take it to a jeweler to confirm it can be resized. He says it can but because it’s going down from a 7.5 to a 5.5 it’s going to have an oval shape to it. I ended up canceling the order. Should I be searching for exactly my size? One down or one up ok? What is the typical rule of thumb. Looking at under 1 ctw five stones