It's a matter of preference really. You have to design your portfolio site to accommodate your visitors or potential clients so this might not please you but it will please others. For example, a client might be looking for someone to design a standard size multi-page menu and seeing the entirety of the menu would be helpful. My best guess is to link an actual size. I link PDF's for most of my work.

I'd rather see full-size images than cropped and shrunk previews. I'd want to see your work at the size it was presented. If you made a website I'd rather see a link than a screenshot.

That might be tricky for outdoor work.

Then get creative . If it's a billboard, get me a shot from the highway. This is just my opinion, anyhow, I'm not hiring.

I would be including the link as well as the the preview screen shot. I think I am going to go with a larger image for the pieces, as well as link to full size when applicable, and/or link to live websites.

Nice photography enhances the appearance of any print based work. If you are a photographer, know a photographer or have some cash getting your work photographed is a great way to present it on both your website and portfolio. You can even leverage the ability to hide things which might otherwise reduce the quality of the work. Beautiful photography can go a long way regardless of the "actual" quality of the work when seen for real.

With websites you should always provide a link to the site if live. Its common practice with websites to provide screen shots just to give the person looking a quick glance before deciding if they actually want to visit the site. Even if its just one that is quick enough for someone to decide if they actually want to visit the site without wasting time in the first place.

What you should NEVER do is only provide a PDF of your work. This comes of lazy and does not provide a quick way for others to see your work. However, a PDF link alongside photography, images, etc is perfectly acceptable. People don't want to be forced to download a pdf and most likely won't unless the initial impression of the site is of a high nature. Then people might consider it but otherwise your kicking yourself in the @ss.

The same can be said for your resume. You should have a easily accessible HTML version alongside a PDF and MS Word file. You would be surprised how many people just want to see a bland word doc over a PDF. So having those easily accessible is a good decision.

lastly, people expect some type of explanation for the work. Even if its only two or three sentences – something that quickly makes it easy for a person to deduce whether or not the work is successful not only on a aesthetic but logical level as well. Many companies also like to know your role in the project whether or not it was collaborative or an individual effort.

I forgot to mention that the majority of my work is photographed, with the exception of websites. I completely agree about photography enhancing the overall quality of the work. That is another reason why I am a little unsure of what size the images should be.