C Parameter In SVM

Paramerter C:

Large Value of parameter C => small margin

Small Value of paramerter C => Large margin

How should you choose the value of C?

There is no rule of thumb to choose a C value, it totally depends on your testing data. The only option I see is trying bunch of different values and choose the value which gives you lowest misclassification rate on testing data. I would suggest you to use gridsearchCV, in which you can directly give a list of different values parameter and it will tell you which value is best.