My suggestion would be that if you're sending it for the first time, then send only a sample portfolio with selected projects instead of your whole portfolio.
And for your sample portfolio just select about 3 or 4 of your best projects, and then a couple of the best images from each of those projects to make the sample portfolio.
Remember that it's only a "sample", which means that you're only giving them a taste of what you're going to offer and hopefully it's a taste of the best stuff.
So with that in mind you probably want to concentrate on images that express what it is you most strongly want to sell about yourself to the potential employer.
In some people's cases that might imply documentation and hence CD set samples and working drawings. In other people's cases it might be renderings and photos of physical models to exhibit one's model-making skills.
In other cases still the person might want to express the fact that they are a jack-of-all trades and maybe include a little of everything.
That's all fine, as long as you realize that it's only a sample and you don't have to include everything and you probably shouldn't do so.
Think of it like your resume, where you're trying to fit as much relevant information into 1 or 2 pages maximum since you realize the fact that the person who'll be looking at it likely won't have time to go through it with a fine-tooth comb and will more likely just peruse through it.
So you put in your best stuff and hope it captures their attention enough to want to call you in for an interview where you then can show a full portfolio and more elaborate resume in person.
Just remember that the resume and portfolio you send them initially tells them as much about you as does the content in it. By this I mean, how it's designed and organized (which tells them about your creativity skills and organizational ability), how it's edited (which tells them about your judgement) and what stands out about it (your originality and what makes you stand out in the crowd and from the piles of resumes they probably already receive).
As for the size issue, assuming you've selected all your best stuff and edited it and it's still too large (file-size wise) then you should consider uploading it onto a free or cheap online website portfolio share service like Issu or LinkedIn's Slideshare among many others that accept pdf file uploads and then all you have to do is send them the link to it.
If they can receive emails then chances are that they can also browse the internet and review an online portfolio and possibility even print from there if needs be.
But I would first take the challenge of figuring out why your "sample" portfolio is still to big to conveniently send over email because that basically tells you that you either have too much information that you don't need to be sending them or that you haven't properly formatted the ones you have there well enough to allow you to send it. conveniently.
Again, this tells them a little bit about how good you are with image editing software as much as it tells them how good you are at editing your content.
There are some online resources and forums where you can find some recent graduates and and working individuals posting their own sample portfolios for suggestion or just in general, and these can give a a general idea of what to shoot for (or not) and at the very least, what you're competing against out there.