'이따만하게' is not a standard word, but '이만하게(=이만큼, 이만한 정도로, 이만치) is as an adverb form of '이만하다 + -게'. '진짜 공연장이 이따만하게 크고...' (the stage in the concert is really as big as this ...) ***이만하다: 상태, 모양, 성질 따위의 정도가 이러하다. (be like this in sizes, shapes, and characteristics, be as --- as)