Are you absolutely positive about this? Really soft equates difficulty in penetration, really big usually means it's not going to be compatible with most O-rings and the combination of the two is going to result in a sloppy, droopy harness leaving you with little to no control. All of the Vixskin dildos are pretty soft (the O2 line is pretty rigid) and most of the Tantus stuff is pretty soft.
This one is pretty tiny
and may not be what you're looking for at all but that's pretty much what you're going to find out there. If we still carried it, I recommend the VIP Super Soft.