What size wire & type/size conduit needed to run connection from 17.5Kw generator to 100A subpanel? Route from outside into crawl space and back inside garage to 'backup' panel?

Depending on the distance, you will need at least #4. Is there a breaker on the generator? At full power / load, your generator can handle about 73 amps @ 240 volts.
Distance and voltage output will change this a bit, but 80 amps is the most your generator will handle. You will need 4 wires and at least a 1-1/2 conduit. 1 1/4 will work, but any bends and you will regret the choice.

Thanks for response. Distance is approx 100ft. I will run two 1.5 inch conduits for my electrician - I am just trying to save money by doing the grunt work. The portable generator is Generac 17500E with breakers on all circuits. For the conduit do I need metal or plastic?

Why is it when people have an electrician hired, they need to ask question here?

Conductor and wire sizing should be done by the electrician that is responsible for the project.

Do not misunderstand me, we are here to answer questions, and under the premise that people are trying to do a project on their own.

The answers given here are based on basic installation practices. Being on site and being able to make decisions based on actual situations may change any advice given here.