How do I define Sequelize.STRING length?

As it is mentioned in the documentation, use:

Sequelize.STRING(32) 

First, I think you need to rethink your design just a little bit. The basic point is that length constraints should be meaningful, not just there to save space. PostgreSQL does not store 'A'::varchar(10) any differently than it does 'A'::text (both are stored as variable length text strings, only as long as the value stored, along with a length specifier and some other metadata), so you should use the longest size that can work for you, and use the lengths for substantive enforcement rather than to save space. When in doubt, don't constrain. When you need to make sure it fits on a mailing label, constrain appropriately.

Secondly Dankohn's answer above:

var Profile = sequelize.define('PublicID', {  public_id: {  validate: { len: [0,32] })

is how you would then add such enforcement to the front-end. Again, such enforcement should be based on what you know you need, not just what seems like a good idea at the time, and while it is generally easier to relax constraints than tighten them, for string length, it's really a no-brainer to do things the other way.

As for using such in other applications, you'd probably want to look up the constraint info in the system catalogs, which gets you into sort of advanced territory.