What do you mean by not recognised? Using HDToolbox, make sure in tooltypes that the device=scsi.device. You should then be able to install and format her. *Should* being the operative word. I've got two HD's here that for whatever reasons are completely useless to an Amiga. I wonder if the HD you have has been changed/modified to the point the Amiga doesn't want to play with it.
Oh and HDtoolbox should be able to determine its proper size, assuming you can get it to cooperate. Just click the 'read drive config' gadget or whatever it is called.