已知向量$\overrightarrow{a}$=（2，﹣1），$\overrightarrow{b}$=（1，x）．若$\overrightarrow{a}$⊥（$\overrightarrow{a}$+$\overrightarrow{b}$），求|$\overrightarrow{b}$|的值.

阅读理解
(Mr. Zhang is showing Wei Fang and Li Lei a modern factory.)
Mr. Zhang: You can’t see many workers here, but you can see a lot of robots. Look, that robot has two big hands.
Wei Fang: How strong it looks!
Mr. Zhang: Yes. It does much work for us.
Li Lei: There are so many big boxes here. What are the robots doing?
Mr. Zhang: They are making fridges.
Wei Fang: How fast they are working!
Li Lei: How much time do they need to make a fridge, Mr. Zhang?
Mr. Zhang: About five minutes.
Wei Fang and Li Lei: Only five minutes! What a modern factory it is!