I'd never heard of E beads, but I heard you mention that you use them regularly and it seemed to be a specific size. But I just found this info you might want to see.

Seed beads are sometimes called E-beads. This refers to the shape of the bead. It is not perfectly round (like a druk bead), but rather is sliced like a loaf of bread gets sliced -- which gives it a squared round or roundish shape. Picture the "E" being the shape of the slicer, as it presses down on a elongated tube of glass.

E-beads in the 5/0 - 6/0 size are often referred to as E-beads, while other sizes of seed beads are not. This is probably to differentiate these from perfectly round beads -- which are not often available in sizes smaller than 2-3mm. If a pattern calls for an "E-bead", with no other delineation, it usually wants a size 5/0 or 6/0 seedbead.

From this site: http://www.landofodds.com/store/seedbead.htm