frame grab quality question

Hi,

I am having an application created which allows users to view a sports video and capture frames. I would like to have the frame grabs as high resolution as possible for printing 5x7's and possibly 8x10's.

My question is, what format would be best? The application has been started and is being developed in flash. I heard that with Hi Def we should be able to get almost a 2MB file? but the samples I have seen (using .flv video files) are all smaller sizes.

What would the recommendations be to grab hi res frames/photo files?

thanks

john