You're going to want about 24 inches of substrate, so you may want to make the base two feet tall
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Also, from personal experience, melamine is AWFUL to use for monitors. I built my raise up enclosure out of it, and it started rotting within a few months from the humidity and soil. Your best bet is to use plywood with 2 x 4 reinforcements, and either seal it with something, or line the part where the soil will be with frp or similar.
The links Wayne posted are very informative, so I suggest you give them a good read. I wouldn't go any smaller than a 8 x 4 x4 though and bigger is always better.