What is the ideal Still Size for Final Cut Pro?

Use the same size photo as your timeline. You can use Photoshop to change the image size. I would make sure to keep the width (over the height) the same if it doesn't exactly match the aspect ratio of your timeline.

I am suing stills in my Timeline.

Not sure what legal means you are using to sue your stills..who's your lawyer?

Okay now seriously, your Canvas crap is because it's just a low res proxy. Spit out a self-contained movie and look at it in Quicktime or better yet look at it through a broadcast monitor for a more accurate representation.

In terms of size you want to keep your total pixels to under 4000. And if you're not making moves on the stills then make them the same size as your sequence.

Yes, unless you want to pan across the stills then you might want it to be bigger. If all you want is to just use it full screen then 720 x 480 works.