Transatlantic, and pretty much anything by Neal Morse (especially his solo albums, and Spock's Beard's first six albums). Lots of long prog epics.
Also, The Flower Kings, well known for some great long prog epics, ranging from the 10-13 minute range, as well as a few 20-25 minute epics, and their 60-minute masterpiece "Garden Of Dreams".
I'll also throw in The Tangent. While not known for their epics, they have had some great instrumentalists through their ranks, and it seems like every album has at least one prog epic in the 20-26 minute range.
-Marc.