Choosing a window size for DTW
If you have enough data, use cross validation.

If you don't have a lot of data, use cross validation on a similar dataset, and transfer the window size (the UCR archive has a bunch of similar dataset)

Don't forget, that the best warping  window size depends on the amount of training data. As you get more data, you can have a smaller warping window, see fig 6 of http://www.cs.ucr.edu/~eamonn/DTW_myths.pdf

eamonn