Big enough so the media bubbles, not so big that it tumbles. Really, you'll be lucky to get one just right, so you'll likely want to get a decent sized one (say MJ-1200) and put a valve in-line.
Truthfully, I don't know what the exact results are... I know I have phosphate in my system and it's just another tool to help fight it, along with light feeding.
I change the Phosguard probably more than I have to. I use filter floss at both "ends" of the media for mechanical filtration, and I change everything in the reactor as soon as that starts to look dirty. From bottom of the reactor to the top, it goes:
Floss--Phosgard--carbon-- floss.