Foam core is your friend. Wood is lovely and durable, but adds a huge amount of weight to slog around with your boyz. MDF board is sturdy enough, even in a 3' square sheet, not to deform while carrying. 3x3 is, of course, too large. Pick a size and shape that is easy for you to carry and maneuver (in potentially confined spaces). Not all tournaments are in big halls and accidentally flattening someone's army with your display board is a tad embarrassing. And if you do shape your board, make it rounded so that you don't hook it on anything while carrying. Even if you go with a rectangular board, rounding the corners is a good precaution against accidental hangups while carrying.
So once you've got your shape foam core is great for ground cover because of the ease of cutting circles for placing your boyz. Don't forget to PVA the holes before painting (if you spray).
To come to a good size I suggest laying out a sheet of paper (newspaper should do). Set up your ladz in a pleasing arrangment that allows you to see them all and give a clear organization to your army. Leave a little space to add interesting terrain bits between units. Then use a marker and draw around every piece. (You can use this sheet later to makr the holes to cut on your board.) Once you've got it all marked out you can use a straightedge to draw lines for the edge of the board. Voila --- you now know how big a board you need.
That's all the simple stuff (read as: mistakes made) I can think of right now...