ReportViewer 2008 switch to different page sizes

This isn't really a problem - the different renderers are rendering the report appropriately for their output. The web viewer is optimised for screen-based reading and generally allows more content per page than the print renderer does as the print renderer is constrained by the paper size that it formats to. Thus you get more pages when rendering for printing than web; however, the content of the report is exactly the same.

The best illustration of this is the Excel renderer - the Excel renderer renders the entire report onto a single worksheet in most cases (for reports with grouping and page breaks set on the group footer it will render each group on its own worksheet). You wouldn't want the Excel renderer to artificially create worksheets to try to paginate your report. It does the appropriate thing which is to include all the data in one big worksheet even though that may be logically thought of as one big "page".

The web renderer page length is determined by the InteractiveHeight attribute of the report (in the InteractiveSize property in the Properties pane for the report) but the interactive height is an approximation rather than a fixed page break setting and your page breaks may still not conform to the print version even though the InteractiveHeight is set to the same length as your target page length.

See this discussion of rendering behaviour for more information on why what you are trying to achieve isn't achievable.