They are the same size as the Incendiary type pellets used in the 8,8cm BrSchr, exept these -I was told- were hexal filled, so not Incendiary but of the explosive type.
As far as I know they were used only in the 10.5cm shells.
I have a picture of a 21 cm Wgr. 42 Rocket head I took on a visit in Germany a few years ago, filled with the same Incendiary pellets as the 8,8cm, however it cannot be published as it shows live ammo. It was meant to be used under the wings of the Fw-190, one under each wing.