Question

why mahabharata is considered a colossal epic

Answer 1

Because it was the largest war in history which lasted for over 13 days. Two armies fought one of which was extremely large and the other was significantly small. Many important people died and even during the war troops were continuously being added. You can make out yourself that this was a colossal war. It was also epic because his almighty Lord Krishna was himself involved in this war.

Answer 2

The Mahabharata is the longest epic poem in the world and many a times described as "longest poem ever written". Its longest version consists of over 100,000 Shloka or over 200,000 individual verse lines (each shloka is a couplet), and long prose passages. About 1.8 million words in total, the Mahabharata is roughly ten times the length of the illiad and the Odyssey combined, or about four times the length of the Ramayana
Gita, which serves as a spiritual guide to all is a sub part of this great epic..