Ensemble Techniques - Bagging | Subset size
like said in a previous answer, the exact subsample parameter value depends on your data. 

But a usual starting parameter that gets you good results in general, and doesn't hurt the data distribution much is 0.9. 

Taking out 10% of your data at each iteration or newly constructed tree, makes your model generalize a little better. You can try out different variations of the 0.9 and see the results.