Is it possible to change test and train data size when using crossvalind function with Kfold param?
K-fold means that the validation step will be performed k times, each of them using a fraction $\frac{k-1}{k}$ for training and $\frac{1}{k}$ for validation. If you want a fixed validation fraction, choose the number of folds that fits:


90%/10% : 10-fold
75%/25% : 4-fold
etc.


I don't know about Matlab libraries or functions, but if you want to do custom folds, you'll probably have to sample train and validations sets "by hand".