What is the correct mkfs syntax to create mirrored disk files systems? I need to make the file system 20gb. For example:

machine# mkfs -F ufs /dev/md/dsk/d40
size not specified
ufs usage: mkfs [-F FSType] [-V] [-m] [-o options] special size(sectors) \
[nsect ntrack bsize fragsize cpg free rps nbpi opt apc gap nrpos maxcontig mtb]
-m : dump fs cmd line used to make this partition
-V : print this command line and return
-o : ufs options: :nsect=32,ntrack=16,bsize=8192,fragsize=1024
-o : ufs options: :cgsize=0,free=0,rps=60,nbpi=2048,opt=t
-o : ufs options: :apc=0,gap=0,nrpos=0,maxcontig=0
-o : ufs options: :mtb=n,calcsb,calcbinsb
NOTE that all -o suboptions: must be separated only by commas so as to
be parsed as a single argument
machine#


Any help is greatly appreciated. Thank you