(let's) say 就说...吧
used to introduce a suggestion or possible example of something
let's say 的真实意思并不是“让我们说”，而要译为“比方说”，在口语中经常在人想举例子时候会说这样的话，类似“就说……吧”。这个表达也可以直接简写为 say。
看完下面这两个句子，大家肯定能明白 let's say 的具体用法。
I'd advise you to take an useful elective course, let us say, swimming.
Try and finish the work by, let's say, Friday.
Say/Let's say (= if we accept) (that) the journey takes three hours, then you'd be there by two o'clock.
say Let's say for example
say, let's say & for example区别
Say is just short for let's say and both are very casual. For example is more standard and formal compared to let's say.
Say是let's say的简短模式，这两个说法都很随意。For example和let's say相比更标准更正式。
If you mean in a sentence like: "Say you were taller." or "Let's say you had a new car." or "For example, if you had a new car."
如果你说像是（表达假设的句子）"Say you were taller.（假如你长得更高些）"或者"Let's say you had a new car.（假设你有一辆新车）"，或者"For example, if you had a new car.（例如，如果你有一辆新车）"这样的句子。
"Say" seems to be just a short version of "let's say". Both of them would not use the word 'if' afterwards in this sort of sentence. "For example" would, also it would almost always have a comma (,) right afterwards, to show a short pause that the other 2 expressions to not have.
Say是let's say的简短版，这两个词都用不到if在它后面的句子。For example会，大概率也会用到逗号在它后面，为了表示有一个短暂的停顿，而另外那两个表达不需要。