I'm looking for information on reducing the size of a roof ridge beam. It's in a 1970s Wausau prebuilt home and runs the entire length of the home. The roof is panelized, so roof panels rest on top of this beam and the beam has walls under it everywhere except a twenty foot clear span. I'm wondering if I can reduce the depth of the beam in that span from 30" to 15" so it doesn't hang down into the room so far.

Our builders here would want to see a drawing. Including how thick the beam is.
A 30" deep beam sounds very strange to me, unless it was done that way to hit the top plates of the intersecting partitions and carry the load down to them and out to the sides.
You can't just reduce the depth in the 20' span. Some of the load needs that depth on it's way out to the sides. An engineer would be needed for something like this.
Some lumberyards will contact their laminated beam supplier to do the calculations for you, if you give them a detailed drawing.