For the platen, anything that is level, stiff and durable should do. MDF are popular in some countries but 3/4 plywood is common in the Philippines.
Cover your platen with a smooth surface. You can add a lining of 1/4" foam but the trend is towards a harder surface. Laminates are popular with rotary presses and canvas with line or long table presses. Aluminum platens have also been used.
Platen sizes pretty much depends on the size of shirts you will be printing more often. 16x19 is a good general purpose size and so is 12x19. I use a 16x24 platen. Shaped platens have better aesthetics and do offer some advantages but there is no problem with a rectangular platen. I used to have a platen with head and shoulders but my platens now have a simple triangular shape that tapers towards the neck. This makes my platen a pentagon. On a rectangular platen, you can mark it to show its vertical center so the shape is really not an issue.