I would say, depends on your case or how you will carry them.
I have a case that I can fit no larger than 8.5x11 and it had clear pages.
My other case can do 16x20 and all of the print in that one at 11x14 matted. No pages.
I think nothing smaller than 5x7!! 4x6 is too small.
Like above, mostly 8x10s. I think those are best. There isnt a standard unless whomever you're showing (employer) wants certain sizes. There isnt a point for an 11x14 if the print looks good as an 8x10.
Plus if a print is too big you cant see the whole thing at once.
I was told that glossy makes a sharper image. I agree to it.
I really like white borders but I don't know if having them is a good thing unless you print it in your own darkroom because you don't have much of a choice.
As for what to submit, I suppose it depends if it's a job you're applying for. I so, then I would treat it like a resume, just add what is appropriate. If not, show your best work (variety of lightning, subject, pose, camera angles etc) and whatever fits in your case. if you don't have a case, then get some sort of presentation book with clear pages that fits yours images.
Thanks all I can think of. Have you tried searching on google? You might find examples.
P.s. I remember being told that having a mixture of sizes is good. Why? I don't know. personally I prefer all 8x10s unless theres something you don't want to enlarge.