Although Go can be played on the floor, most people will expect to be able to sit at table and chairs to play. Of course if you have a demo board on a stand and space to do it, then some people will like to stand up and play on the demo board, or watch others do the same.
You need enough table room for usually two people each side to sit and play, or more room if you have more helpers, and room for anything you are displaying and for the leaflets and so on. The minimum for two teachers is a table 2m to 3m long. Longer than this and you will feel lost. However if you have lots of things to display - such as a Go ban, computer, sales stock - you will need more table space.
A cloth might be needed to cover the table if it is too old and dirty, or to improve presentation. A suitable old sheet or blanket would do, or the organisers may provide rolls of paper.
Chairs should be a minimum of two each side, but often families or couples turn up and some want to sit and watch one of them play, so more chairs are useful. Beware of other groups taking them if they are temporarily not in use - guard your territory well!
Additional stall space will be needed for a demo board on a stand or for display boards of photos etc.
If your stall is against a wall then you have the advantage of being able to put up pictures and signs, but check what fixing medium is allowed. If you have a computer, then you will probably need to make sure there is a power source.