Hi Colin,
Have a look at this website:
Goo Systems Projection Screen Paint
They do paint which is designed exactly for this purpose rather than go for any home-brew conconctions which may or not work well!
For the borders, you could used any Matt Black paint from B&Q say, but to get the best effect, use a black velvet material - you can get sticky back velvet on a roll from here:
Fablon Sticky Back Plastic Black Velour (450mm x 10m) 65217 Adhesives Glue And Sticky Stuff Sticky Back Plastic
I got a roll of this and it's a very deep black.
The other alternative is to get "Goo Flok" which is a sticky backed black flock that Goo Systems above sell, again just for that purpose. You can either buy from Goo Systems or go to the manufacturer direct:
:: Welcome to British Felt ::
Ask for Goo Flok - they sell by the linear meter. again, I got some of this and it's a great product - more versatile than the fablon stuff in that it has a much better adhesion and is more flexible, but to be honest, sticking to a wall, you don't really need that flexibility, so the fablon would be fine..
As for the width - if you're creating a 16:9 screen with a 16:9 projector, use about 3 inches and that'll be perfect.
If you're creating a 2.35:1 screen and using a 16:9 projector - so you'll get black bars projecting then use 3 inches down the sides and about 7-10 inches or so (depending on the size of the bars) top and bottom.
Mark