Building an illustration portfolio
Creating an illustration portfolio is easy—you choose your 10 best works. My recommendation is to not go overboard and not print over A4 size. Why? Because you will need to take the prints to conventions, festivals, fairs, or whatever you have in your country. In those first periods of your career, you will be a door-to-door salesman, so you need to have all your work nicely packed in a container that is easy to open and shows the illustrations. You will probably need to show the illustrations while standing, so if your works are too big, this will create problems for you and your interviewer.
A little word of advice: what you're selling is your marketability, meaning that if they can put you in a category, it will be a little bit easier for a company to hire you or request an example of your work.
For example, my forte is cartoonish illustration, and that's something I can't change because I spent 10 years training in that...