I started a discussion on this a year or so ago, but was never posted into the official Army Lists by Rich...
Not played them yet tbh, somehow got distracted by yet more shiny...
28mm, using the excellent Perry Miniatures figures.
I don't, as I went 15mm for my ACW stuff. But if I were considering 28mm, the Perry stuff would be the top of my list. Their sculpts are pretty uniformly amazing. If you want period fencing, split-rail fences were pretty common to the period and are easy to make. Also, I posted a tutorial on making dry-stone walls in the "Miniatures, Modeling, & Scenery" sub-forum that might be of interest to you: viewtopic.php?f=19&t=10109
The ACW plastics are definitely the newer style plastics, being far more evenly proportioned than the old plastics. I can't say how well they scale with the metals, but my assumption is that they'll be a little slighter.
Germane to this topic, I just picked up an interesting book titled "A World on Fire: Britain's Critical Role in the American Civil War" by Amanda Foreman. I need to finish the Chickamauga trilogy I'm currently chewing my way through before I'll have a chance to dive into it in depth, but I wanted to mention it as it pretty much directly deals with the subject of this thread.