1. Used to show your agreement with someone who has said something correct
"So you mean you won't come to the party with me?" "Yes, that's about the size of it."
2. An approximately accurate version of a situation, event, or circumstance.
整个形势或情况就是如 此, 全部都说淸楚了（一般用于谈话的总结、结束）
Well,that’s about the size of it！See you tomorrow. 好吧，整个情况就是如此！明天见。
That’ s about the size of it. You’ve understood it perfectly 一切都说清楚了，你已经完全明白了。