I was amazed at the size of the solar power plant.
I'm so surprised it caused me to sigh. This is like when you run into a movie star, and you go in surprise shock. Then, you breathe out in a sigh because of your great amount of admiration and wonder for this person.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.