I've used a couple of workarounds in the past to deal with this. Not the most elegant solutions (I suspect there may now be easier options), but workable.
One workaround is to alter the display extent for that page so that their is enough space for the overlapping legend to display without covering up the data. In Data Driven Pages, use the Data Drive Scale option and enter an appropriate scale that works for the maps that are in issue. However, this can be "hit or miss", as doing so may cause surrounding areas to be "missed" as the series is created, so check the output. See the Optional Settings (Map Extent) section in this link:
ArcGIS Help 10.1
Another option is to alter the grid you are using as an index so that the pages will automatically be sized as you want to accommodate the legend issue. This will require manually editing the grid, which can get messy if you have many grid polygons surrounding the one you want to change. Two further options for this approach - one is to "overlap" the problem grid (so the legend covers up information displayed on the previous page). Another option is to expand or shrink the grid for the problem one(s). What works best will depend upon your data and how it displays.
Be aware that if you use any of these options, it will impact how the viewer sees the results. It may entail having to take extra steps like calling out to the viewer that the "problem" map is of a different scale or extent than the others in the series. So unfortunately no easy "one step fix" solutions.
Chris Donohue, GISP
