Hello there,
Welcome to the Forum..
Look's to be about four-foot square (1200mm or 1.2m in new money)?
We need a little more information, really..
How thick it is? - This will determine what size framework you will need under it.
Going to exhibitions: So I assume it will be modular, in some way?
Presumably, it will not be kept/used outside? - So plywood should be OK.
You also need to consider how big each module will be.
How you will build any scenery, and whether the modules will have scenery/buildings permanently on them. - This will give an idea of the weight of each module. Whether it can be a 'one-person' lift, and an idea of how the modules can be stacked/transported.
What is your modelling experience?
How will you transport the layout?
Where will you store it?
How will you support it?
Have you thought about the size of the loco's and stock? - You would be amazed how much room it all takes!
PhilP