JavaScript popup or Constraints & how-to

I am wondering how best to go about providing a uniform input into my database from my users (all of whom will connect via a Web browser). To ensure that only permissible products, sizes, packaging units, etc., are input, would it be best to present the permissible options in a popup (or other) list in the browser? Better to use constraints on the permissible values over in the table editor? Both? Some entirely different method? If constraints are a good way to do this, what is the practical & desirable limit on how many MATCHES can be specified for a field, or within a table? Thank you for your time & generosity with expertise. Daniel