I'm just saying…
我的意思是…

① Are you saying…?
你是在说这个意思吗？

② I'm not saying…
我不是…这个意思。

A: Are you saying I'm fat?
你是说我胖么？
B: No, I'm saying you put on a few pounds over these couple of weeks. You're fatter, but still not fat.
不是，我只是觉得这几周你长了几斤肉。你是比之前胖些了，但不是胖。

All I'm saying is that…
我是说/我要说的是…

All I'm saying is that if you want to buy a car you shouldn't buy the Porsche. I'm not saying you can't afford it. I just don't think it's a very good car.
我是说如果你要买车的话不应该买保时捷。我不是说你买不起，而是我不觉得那是很好的车。

All I am saying is that he is a good guy and everybody really likes him.
我要说的是他这个人特别好，每个人都喜欢它。