Suppose I have 100 positive samples. How many negative samples do I need to have in order to make the classifier work the best. In many papers, I have noticed that they take 4 times or 5 times the number of positive data sample to get the negative data sample. Will such a data set be useful?
I guess you are not limited to these 100 samples. Generate more, and let each 5th be negative. Then reduce number of positives by random removing 4/5 of them.
And check this out Training imbalanced data set
This is small quantity, you'd better have 50:50 negative vs positive.
It is recommended to have around equal number of instances in each class. If not then you should normalize the data. 4 or 5 times more positive class size could produce highly biased classifier. However if your negative class is not well defined then there are ways to train your classes with only one class as mentioned in the paper here - http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2248178/