Saurabh, there is a way you could compute sample size, if you know the size of the maximum standard errors you were willing to accept. Then
1. You create a design, create random data (using Excel or the data generator) for 100 respondents and estimate a logit model. Calculate the the average standard error.
2. For any other sample size N you can multiply those standard errors by SQRT(100/N) and see what size standard errors to expect.
3. Choose the sample size that gives you the size of standard errors you want. For CBC we recommend standard errors of 0.05 or less, and I don't see a reason that makes it different for MaxDiff.
You'll likely need more observations if you do Express. Also, Sparse gives you better data than Express, so for most purposes I'd use Sparse rather than Express.