are you working in HD? If so, 1920 X 1080 pixels will work. I don't know what the resolution is for 16:9 standard Def but you can always half the 1920 X 1080 and adjust the size in FCP.
The point is is to get the photo around the same size as your number of pixels in your FCP sequence and adjust in FCP from there.
It's hard to screw this up so you should be fine.