New build in progress.
Loft storage flooring to be on attic trusses designed to take 150kg/m2. 
No other support required, structurally.

 

Want to add a timber post under a truss, for belt and braces approach to load bearing
Post will sit within a 100mm (or 75mm) wide stud wall.

What timber to use for the post?
Perhaps double up two stud posts?

Standard stuff, especially if it's to help you sleep rather than because a SE says it must be there.

 

Just a stack of standard timber of whatever your wall is made of, e.g. the ridge beam of my house is supported partly by 2 internal walls that have a stack of three 147 by 47mm C16 regularised timbers running up inside the wall.