I cut a semi-adhesive masking film, lay it down on the paper (if self-coating) then coat using the film as a mask. This will give a perfect edge. I have also used Ruby Lith on pre-sensitized paper (not as good though).
I think I read somewhere that you can create your own masks with ruby Lith and acetate. As in create a series of masks to suit you. (The print industry has been doing this for years). Providing the Ruby is on the bottom - as the contact layer I believe it works. I have to say I have not tried it yet though.
I use a commercial printing plate contact frame (with a vacuum) so this gives me up to over sized A3 as an image area.
The previous posts have more than covered your original question though.