Lasso Regularization using Matlab

I want to find the value of parameter estimates using lasso function in matlab. X is my input(which is 20X1) vector and Y is the output(size 20X1) , the output b should also be a vector with 20 values,but I am getting 100 values..dont know how and how should I interpret it !!! Also it is written in matlab documentation that I can specify values of lambda,but no syntax is provided. Plz let me know how to specify values of lambda so that I get required values of parameter estimates