Use of versions, well IT depends. If you are early in the "investigation" process then it does not matter that the diagram is "in motion". But if everything is going well then it might be important to start versioning. Which can be done in several ways. You can create a baseline of the diagram, which is basically a version. You can clone, but I should not use that for a diagram (I think) since cloning serves a different purpose. Of course, you can save the diagram as a PDF, which would be my approach, it is easy.
The largest poster size that Sparx allows is A1. So, I use that for my "canvas". While printing it is easy to scale it down or up.
Concerning the size, I would go for the bigger the better. I use smaller sizes when the discussion is more 1 to 1, but for a group I would use at least A1 size.
It depends, it depends on the group of people you want to talk to. I am a fan of a view Archimate viewpoints though, like Capability Maps, Business Function to Roles, Business Process Overview, and Application Interfaces.
I've used HP Plotters in the past for A0, A1 but they are expensive so have often used a print shop. The printshop that I go to charges somewhere of 17 to 25 Euro ex VAT for a A1/A0 poster. They have the ability to make it even larger. The nice thing is that posters are more constrained to the height (talking in landscape here) then in size. Paper is on a role and cut-off by hand. So, in principle the you can go as wide as you like.
The standard answer is “IT depends”. But most of the time I make the poster diagram "floating" that way it is easy to occupy one screen and put the browser on the other side right (or left) beside it. In many cases it is good to create an overview poster that is a composition of different diagrams. In many cases you do have some diagrams that convey parts of the message. You can reuse them. Though putting them in the same space might mean that you have to reorganise your diagrams for a better fit.
There was a time that mechanical engineers used large drawing boards and we IT-guys used data-flow-diagram and a pencil to draw our stuff on A4. Now IT is getting so all covering that we might need drawing board sized screens.