What size laminated header would I need to use over an 18 foot garage door? And also a 12 foot door?

There is no information here as to what's above these openings which greatly effects the header load in deflection.

When you say laminated are you referring to Micro lambs or just site build 2bys sandwiched with plywood?

Do you have a Mirco Lamb manufacture there. If so They can engineer it for clear span and header load. Here it's a basic with and height to dead load and live load which is snow loads.

Thanks for the help.
I am referring to a micro lamb header. There will be roof trusses above the header on 24" centers. With osb and shingles on the roof. We have no extra weigth from the snow.

opwdvr

What is the room there for a header weight?
Micros are pretty salty. They Basically come 3" wide and you nail a couple together. The weigh a ton so to build the header it may be best to a build in place. The Mirco supplier can give you the load and clear span specs.

They way we do it at times is go to a real lumber yard and get 2x10 or 12s ( 18' in your case and use exterior 3/4 or 5/8 ply and sandwich that in between the 2x10s or 12s to make a semi micro lamb beam. I use exterior adhesive and cut 10" wide strips out of the ply. Nail spacing 12" OC. Start the first 8' long piece and center that in the header. Chase it to the ends from there. Add another 2x10 or 12 over the first applied ply. The next Ply on that second 2x10 or 12 stager that joint from the first ply joint. Ands so on.

Looking from the top down it 1st 2x10 then ply 2nd 2x10 then ply 3rd 2x10.

Sight the lumber and keep the backs up. The bow end up to header load.

I'm not an engineer so you didn't get this from me. Covering liability here. So call Lamb supplier and price it out and I gave an option B

Again I don't know how much room you have from bottom of door header to to sill plate