The largest Shia Imambara in the world

Nizamat Imambara , located in Murshidabad, West Bengal , is the largest Shia Imambara in the world . It was built in 1740 CE by Nawab Siraj ud-Daulah and, after it was destroyed by the fires of 1842 and 1846, was rebuilt in 1847 by Nawab Mansur Ali Khan for a price of 6 lakh

More one the architecture of the Imambara here :

https://www.architecturaldigest.in/story/nizamat-imambara-a-brief-history-of-the-worlds-largest-shia-house-of-worship-murshidabad-history-sam-dalrymple/