CousinBotony - my advice would be to try a few on to see what you think looks nice and don't get too caught up in what width it "should" be.
Also, if you do decided to get an eternity band, I think it's a good idea not to have diamonds/gems the whole way around, leave a little bit of metal so that it can be resized more easily as you're (hopefully!) going to be wearing it for many years and maybe even passing it on so room for it to be altered seems prudent.