已知平面向量 $\stackrel{\rightharpoonup }{a}=\left(2,4\right),\stackrel{\rightharpoonup }{b}=\left(-1,2\right)$.若 $\stackrel{\rightharpoonup }{c}=\stackrel{\rightharpoonup }{a}-\left(\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}\right)\stackrel{\rightharpoonup }{b}$，则 $\left|\stackrel{\rightharpoonup }{c}\right|=$．