This is the 61st most frequent Japanese word.
大
"Big" or "large."
Here, "大" (big) is used as part of the word "大きい" (big), which is an adjective describing the size of the building.
この建物は大きいです。
This building is big.
In this case, "大" (big) is used as a prefix in "大成功" (great success) to emphasize the magnitude of success.
彼の夢は大成功を収めることです。
His dream is to achieve great success.
Here, "大" (big) is part of the word "大人" (adult), where it metaphorically denotes maturity or growth.