Swapping HD to SSD on late 2013 21.5" iMac

My iMac has a 1 terabyte HD with 800GB used. A video recommended I purchase a 500GB SSD drive which I did, but now I'm wondering if I should have purchased a 1 Terabyte SSD to match my existing HD? Before transferring from the old HD data to the SSD is there a way to not copy the whole old HD to the new SSD? Thank you.