Sarah it depends on the shape of the shoe. If the shoe has a good foot shape then you wont need a lot of extra space. The extra space given to shoe sizing is usually accounting for the fact that the toe box is tapered and in order to keep the toes from banging into the shoe they give you extra length. One thing to remember is that as the shoe lengthens it also get wider and the placement of the instep/arch are moving as well so more space can sometimes mean the shoe really isn't fitting right at all. When it comes to swelling that is really what the laces are for. If your foot swells you just need to loosen the laces and you should be ok so I'd make sure the shoe fits right from end to end first and the rest should be ok.
For the sake of recommendations since you are a barefooter I'd say to try the new Minimus Zero by New Balance. It's got a somewhat odd feel for some but it's an overall pretty good shoe. Merrell is obviously there as well but I'm just not a fan of the narrow mid foot. If you're wanting something somewhat "heavy duty" then I love the Altra Lone Peak. It's heavier than some of the other shoes which is a bit of a draw back for some. I've found I like the extra weight at times because it really gives my legs a workout. Inov-8 also has some pretty nice trail options but the sizing for them can be a bit tricky so I'd say to try and find a place to try them on before you buy.