It's a matter of personal preference, but it should fit within the boundaries of 8.5x11".  Some people like square formats, some like elongated, and some like portrait orientation, but that's all up to you.

hope that helps

Thanks!

oh I would never go that small! If you are sending it to them, then sure.  That is a much easier format to deal with. But at an interview, do you seriously want your drawings all tiny? What if you are being interviewed by two people? That means there are three people flipping through a tiny book trying to see things. I have had good success at A3 (almost 11x17"). Many of my drawings and presentations in undergraduate were drawn for the 24x36" scale... and most drawings I have worked on professionally are A1 or A0. Getting all that into a 8.x11" really doesnt do those projects justice. 

Thank you. Even I was thinking that A3 would be the most appropriate size. My only worry was would the A3 be too big for them to handle. If I'd consider two people going through the portfolio at once then A3 would be my best option I guess.