All good advise. Several other things to think about.
1. Weight. Big boards can be very heavy by the time you have all the bits attached to them.
2. Stiffness. A small board needs to be a lot less stiff than a big one because the loads when moving it are less . This makes point one worse. Boards that flex will damage your railway & buildings etc may fall of due to their mountings failing.
3. Awkwardness. Moving big boards is physically very awkward compounding any weight considerations. Your finished railway board will not appreciate being bashed around & your partner/ wife/ mother will not appreciate you banging into things with it.
4. Consider the underside carefully. If the table you are going to stand it on is a family heirloom make sure the table is well padded before you put the board on and make sure there is nothing sticking out of the bottom of your board.
5. If you are going to use more than one board to alleviate the problems above remember that they will need to line up the rails perfectly and every time. As I have never built a layout where this was necessary I will let others tell you the best way to achieve this.
6. Storage. You will need to store your boards as well as use them. By the time buildings etc are involved they are not the easiest things to store in a way that will not damage them.
I would recommend that before you put any railway stuff onto your board & possibly before you even put the bracing pieces in place you try it & make sure that the size works for you. Your can always tape a few cereal boxes to the board to simulate buildings etc. This really isn't time wasted.
Don't let this put you off - it can all be achieved by a beginner. It is just that thinking it through first can save a lot of heartache/ wasted time & expense later. Asking the question was a good step to achieving success.
Chris
Somebody please comment on the best way to join boards