When assembling a book or web portfolio you are given a set of parameters that dictate a smaller playing field, in terms of image size, but one that still offers the possibility of affecting the viewers experience of your portfolio material, if subtly. Size plays a key role in the layout of a portfolio.
Changing image size reorders the visual dynamics of a picture and may in turn shift the picture's “psychology” but often unpredictably. This suggests that the size of every picture, or series of pictures, needs to be considered on an individual basis and with as much experimentation as possible.
As often as not, the pictures in professional portfolios are of uniform size. Uniformity requires planning and that reads as “professional” and if done well may be quite elegant. However, it can be interesting to vary picture size according to image and sequence to create a more visually energetic presentation. Fashion magazines routinely employ such strategies and as a consequence, so do fashion photography portfolios.