I think that as long as the firm has a contact email adress it won't hurt to send a portofolio and a resume, with the cover letter written in the email message. 

Just try to make the written stuff short and concise. And include in your portfolio only the best stuff you have , not everything you ever created ( just saying... i saw a lot of portfolios cluttered with all sorts of irrelevant pieces ) . 

Well that's what i did when i applied and it was ok. 

Hope it helps. 

Having been on the hiring side... I'd say, never send your complete portfolio.  When I'm just scanning resumes & work experience, I'm not going to look through someone's complete portfolio.  I'll look at maybe a couple pages and then I'll feel kind of annoyed that you didn't edit it down to a brief "best of", because it's essentially a waste of my time to get your 30 page book.  

If you have a website, link to that, or send just a few pages with samples of your work on it.  Your complete portfolio is for face-to-face interviewing.  Make sure your email is around 5mb, not much more.  

In the corporate world, they have a thing called an "executive summary"... basically any report that the mid-level managers make, they put a cover sheet on with just a bullet-pointed summary of the report's contents for the executives to look over.  It's respectful of the executive's time, and helps them make decisions quickly.  Think of your pages of sample work as an executive summary, and save the full portfolio for the interview.

Cover letter + Resume + Teaser Page

Three pages max for less than 10 years experience. Cover letter should never be more than two paragraphs long, with lots of white space.

Resume is one page until you have enough experience to justify filling two.

The teaser should be a one-page collage of your best work, finished and process, arranged with the sole intent of getting their interest so they call you and ask for an interview to see your whole portfolio. Text on the teaser should be minimal.

For those who have more experience, substitute a list of selected project experience for the teaser page, ideally with thumbnail images of completed work and brief descriptions of each project and your role on it. This will be a more text-dominant document, and may span several pages as you get to the 20-years experience mark.

And keep the graphic design of the whole thing simple, clean, and readable. Nothing fancy.

I appreciate the input this far...I already have a better understanding. I have a couple last questions 1) If its a small firm with no human resources director, who should the email be addressed to? 2) What should the subject line of the message read? I'm thinking it should be simple but definitely professional.

In that case, address all correspondence to the senior named partner (if you don't know anyone there), or to your personal contact (if you do).

If it's a firm where you can't figure out who the senior named partner is, safe bet is "Dear Hiring Manager".  

if its a small office the names of the firm directors are on the website most likely.  you can address it straight to them.

since this is for internship (i am thinking european or asian meaning here, not north america) keep it simple and short and to the point.  this is a good idea for anything you do from now on.   sometimes we get crazy long mails that require us to scroll down a few pages.  Those ones tend to be narcissistic and we often get bad vibe from that.  a few sentences is enough. 

say clearly what you are looking for, how long the internship should be, if it's part of a year out, or part of the post-graduation requirement (if you are in europe) for experience, etc.  Also be clear about when you are available to start and if/when you need to return to school or what you have in mind for next steps if it's important to you.

as for the portfolio, a teaser page is annoying for me, personally.  i prefer a few pages to see projects with a bit of depth, and what kinds of skills you have.  less than 5mb is spot on in terms of size, but in our case we have definite desires for design sensibility and production skills, so take as many pages as you need if it gives a better picture.