which fitments do i need? oil cooler

[stevo clio]

im going to buy an oil cooler for the 5 engine but planning on using the original pipes from the sandwhich plate but which size fitment do i require?

http://www.rallydesign.co.uk/product_info.php?cPath=340_367&products_id=2256

options are 1/2" 5/8" and 3/4"