I need to know what size of lumber to use to free span acrss 16'6". It is an interior wall inside a new shop for a loft

A little more info needed

Is there going to be a load on top of this span?If so,how much weight.

How long is the loft,I see 16'6" wide.

What is below this span,is that the wall you spoke of?

you'll need a microlamb beam, you'll need to call a homecenter and get in touch with a structural engineer to tell you the size beam you'll need. My guess is a double 2x 16 by the length you need