kiwis: i have win 7 ultimate 74bit

what size hard disk should i put this on?

 
I would suggest 2 drives:
"C" drive for the OS - 120GB - preferably a SSD
"D" drive (HDD) for data - at least 1 TB (3TB is currently the price "sweet spot".)

If you only want one HDD, I would suggest partitioning it into "C" and "D" drives as above - HDD size 2TB or more.