Thank you for your input!
The width of the report is not limited to fit on a single sheet of paper. In case you design your report wider, or there is an item that will expand the report in a horizontal direction (like crosstab with dynamic column groups), horizontal paging will take place and part of the content will go on the next physical page when printing or exporting to PDF for example. This is why for report on-screen preview, all report viewers support Interactive view without a horizontal split of the content (http://docs.telerik.com/reporting/report-viewers-interactive-print-view).
For convenient design-time experience, we provide a visual aid which shows exactly what you need: the horizontal limit of the report content on a single sheet of paper equal to the paper width (height if you use landscape layout) minus the corresponding page margins. Please find the attached screenshot. It is up to the report author to set the desired report width. 
Displaying the height of the report at design-time makes sense only for reports that will not grow vertically, like reports data-bound to a single row of data. In that case, it seems more appropriate to me to have the same visual aid in vertical direction also. What do you think?