Arc sec/ pixel is a measure of the plate scale for your set-up.
Based on effective focal length and the pixel size of the camera...
If the object you want to image is say 20 arc/sec in size and the arc sec/ pix is 0.5 then the object would cover 40 pixel on the image.
The optimum arc sec/ pixel really depends on your seeing conditions. Somewhere around 0.5 arc sec/ pixel is a good starting point.
To answer you other questions download CCDCalc and enter your data - it will give you arc/sec pix (plate scale), field of view and comparison with various objects.
I've entered your data into CDCCalc and get 0.19 arc sec/ pixel (Ignore the telescope label - I entered 250 f5 with x5)
HTH
http://www.newastro.com/book_new/camera_app.html