I picked up a few Hovels 15mm Civil War buildings as well as some Kallistra 12mm Civil War stuff to see how they match up with Warlord Games new "epic" scale Civil War.
I haven't gotten the Kallistra stuff yet, but the Hovels were delivered the other day.
As you can see by the Hovels pics., they are darn near a perfect match. I put one of the infantry sprues up against the doors of the buildings, and the individual soldiers are the perfect height for the doors. They would walk through and their heads would just brush under the upper door sill.
So it seems that Warlord's epic scale is 13.5mm or thereabouts. And it also seems that it means that Hovel's buildings are not true 15mm either, but slightly undersized.
Which means that any 15mm ACW ranges out there that are on the larger side are actually a bit too large for Hovels 15mm. There are at least two or three ACW 15mm ranges I know of where individual soldiers would not come close to fitting through Hovels' doors.
Also needs to be mentioned that some Youtube gaming channels have described this new "epic" scale as being like "glorified 10mms." They are most certainly NOT glorified 10mms. They are closer to 15s than 10s.
Kallistra's 12mm will probably match up to these very well, particularly things like their casualty markers, limbers and horses, artillery pieces, etc.
Also one final note with regards to Antietam Bridge. That bridge is actually larger than many Civil War buffs think it is. It is 124' long and 12' wide. As you can see with the tourists on the bridge here, Warlord's epic scale soldiers are actually a closer match to the scale of the bridge than some 15mm ranges are.