hd size for quadra 840

i've got a quadra 840 here with a 1.5 gb hd and want to get openbsd onto
it. after reading the install instructions i'm to understand that i need
a 2nd disk to install openbsd onto from the onboard one that's running
mac os 7.5.5. if this is incorrect, do tell.

what is the largest hd that such a machine will accept? i've found one
that's an apple-brand 50.1 gb 50-pin drive.

cheers,
jake