I looked for internet sources without success, however I recall seeing an image of an intact tablet and it was about 30cm square. The writing on them was very small, as in the attached image of a recently-discovered (2015) fragment of Tablet V containing previously unknown text from the Epic. The words appear to be about the size of individual letters in modern printed pages (plus the scribe uses the entire surface of the tablet with words packed together from one edge of the tablet to the other), so it's no surprise that the entire Epic could fit into 12 tablets.