>what size?
I don't think it matters much what exact size you make. Something around 20" by 30" should do the trick. Because you're probably going to make mistakes, I would keep them cheap.
>how many?
If your speech has an intro, three body sections, and a conclusion, then 1 board per section is a good intro number. 5 Is a good place to start
>where does everyone get the bags they use?
Something like this should work
General Advice:
Write your speech before you start working on boards. I know boards are fun but they are also more difficult to change.
Storyboard the boards. take a piece of paper (or just use Google Slides) and with a pencil draw what you want to appear on the boards. This will make it easier to work on the boards themselves with a reference.
Unless you have a very clear vision, it's better to keep the boards minimalist. One or two pictures per board are all you need.
Edit: Fixed Spacing