I really suggest you get yourself over to FredMiranda.Com and pick up his WPPro (Web Presenter Pro) plugin. It only costs about $20 or so and it's worth every penny.
Using it you can downsize your image to anything, AND apply slight sharpening at the same time. Makes that part of the job real easy.
Another CRUCIAL element is in how you use Save For Web... First you must convert your image color profile to sRGB (that's assuming like most folks you're using AdobeRGB for your own editing). Then in Save for Web, you MUST tell it (one time, as it's a remembered setting) to embed the color space. Doing that involves two steps:
First of all, click on each image (most of us use the 2-up display with the original at the left and the output image at the right) and then using the small triangle "thingie" in the upper right, select the Use Document Profile (I'm writing this from memory and haven't looked at it in awhile, so the exact wording might be different, but you want the one that uses the document color). Do this for your original AND for the target. Second step is to select the checkbox in the right-hand dialog to use ICC profile.
Unless you do both those things, the color profile is not embedded in the resulting image and you have NO guarantee that the viewer will see the colors you expect them to see.
About sizing your image, you must learn what image sizes the web site supports. This could be specified in two different ways: First is actual pixel dimensions by pixel dimensions. For example some sites require that you limit the image to 600 pixels on the long side. The other way they might specify the size is to say the image must not exceed 100k (or some other value).
Model/Photographer portfolio hosting sites are all over the map with those sizes, and indeed the limits also vary for differing membership levels, with a free membership having the smallest limits. If you're trying to give your model (or yourself) the best possible quality you need to become familiar with what those limits are for the site you're targetting.
If you don't size the image properly and the site even accepts it (some simply reject the upload) then the site may automatically reduce the image to fit the size limits. This is the worst possible thing that can happen to your image as the resizing methods are nowhere near as good as what you could produce.
Having said all that, I try to find the best compromise between size of the image and what the MAJORITY of viewers can see without having to scroll around, and I also try not to use images large enough to make it easier for someone to rip off and print a decent size version for themselves. So the limits I usually use are 700 pixels wide for a landscape orientation image or 600 pixels high for a portrait image. For me that's the best compromise.
Hope this helps...