You need a descriptor, "r0", and the device driver, "ram". Look in
/dd/cmds/bootobjs to see of you have them already. Its possible that
they are simply not included in your boot file.
The size of the ram disk is set by adjusting the Default Sectors per
Track Value. In my /r0 descriptor its set to 6000 with a 256 byte
sector size, giving 1.46 MB of space in the ram disk. I guess you'd
alter the ram disk size by using moded to change the number of sectors
and saving the result.
If you haven't got these files, e-mail me and I'll send you a copy of
my /r0 whichever files you need.
Use os9gen to make a new boot file with the descriptor and device
driver added to your existing module list.