My channel set 1/2 eternity was sized over 2 sizes by the store. It was in stock and so I didn't have to wait they resized it. One stone they said fell out during sizing but they replaced it no problems and I've been wearing it 7 years w/o any loose stones. No odd oval shape either.
In general my trusted local jeweler says 1 size is almost always doable on a non full eternity. So I always disagree with those here who say less. But I have small fingers and have had to have a ton re-sized over the course of my life and have asked this question a lot.
I always say ask the jeweler as they know what they can and cannot do, but usually I find 1 size has always been doable. If I go for 2+ it is hit or miss with what can be sized.