On Saturday, 23 July 2016 16:35:22 UTC+1, Tim+ wrote:
I've spent a couple of days removing manky old lath and plaster, rewiring
and replumbing.

https://www.dropbox.com/s/gw64fjiz7v...2013.jpeg?dl=1

My daughter was going to get a man in to plasterboard the wall and build a
new door frame for a new back door but having gone this far I'd like to see
it through myself.

I've not plasterboarded myself but can't see that it's gonna be too hard,
particularly if it gets a plaster coat on top.

Given that a) I might be working alone and b) the ceiling is 9ft high,
would I be best using 6x3ft sheets of PB?

The idea of staggering joints was that the abutted boards crack in time and will do so more easily with a matching line all the way up. Just leave a wide enough gap between the boards to allow you to press plaster in between when you plaster (a nail width is plenty) the heads of clouts will serve to hold the board in place while you get a fix in. Then the modern fibreglass tape should prevent cracking -or not as the case may be.

Would I stagger the joins across wall using a 6 + 3 ft piece followed by 3
+ 6 ft piece?

For a kitchen/dining room (well away from sink & countertops) what
thickness and type of PB should I use?

You are governed by the imperial measurements used in the construction. What size are the studs and how far apart are they?
Then work out how easily you can cope with the boards.

All you need to do to overcome the studding problem is insert an extra stud every so often. Alternatively you can put timber across the studs and use any measurement to suit yourself.

The thin boards are lighter and less able to cope with bending. But how big is a wall that you have to worry bout a sheet breaking occasionally?
Can you get it in the room no problem?
You can buy huge sheets and cut them to suit outside if the weather is nice.. But the more expensive smaller boards will make new experience more pleasant, it may even be fun.

Either way, a disaster will be slightly easier to cope with, well worth the cost.

Just cut and place every board ready for screwing in place. Put all the wiring in first and the insulation etcetera. Keep them 1/2" off the floor to stop damage from floor-boards bouncing. That is all there is to it.