Drawing a rough box on a map of the US I estimated about 40 degrees of longitude by 20 degrees of latitude. The most detailed maps are the 7.5-minute quadrangles, which I believe are available for nearly all of the continental US. Since 7.5 minutes is 1/8 of a degree, that gives 40208*8, or a bit more than 50,000 maps. A quick browse through the USGS site found this quote (http://store.usgs.gov/b2c_usgs/usgs/zproductinformation/%28x...): "It takes about 57,000 maps to cover the conterminous 48 States, Hawaii, and territories." They go on to mention that Alaska's most detailed coverage is the 15-minute quadrangle series. A similar estimate for the 15-minute coverage of Alaska is around 3000 maps, plus 1/4 of 57K (14K) for the 15-minute maps of the area already covered by the 7.5-minute series, plus a few thousand for the smaller-scale series (30x60 minutes, 1x2 degrees, index maps, etc.) So I'd guess somewhere around 75-85 thousand maps.
The majority of these maps were originally intended to be printed on 23x27 inch sheets of paper. Assuming they were printed at around 300 dpi, somewhere between 1-8 bits per pixel (after compression, this is the shakiest part of my estimate!), that's around 50 million pixels per sheet, somewhere around 6-50MB per sheet. So somewhere between 1/2TB to 4TB for the whole data set (and I could easily be too low or too high by a factor of 2).
Heh, looking at my neighborhood, the smallest available file is a scan of the 1933 7.5-minute Sierra Madre quad, 5.24MB, and the largest is the 1979 30x60 minute Los Angeles quad, 44.53MB. Most of the 7.5 minute quads seem to be in the 15-30MB range. So I don't feel so bad about my "shakey" estimate of individual map sizes!
The majority of these maps were originally intended to be printed on 23x27 inch sheets of paper. Assuming they were printed at around 300 dpi, somewhere between 1-8 bits per pixel (after compression, this is the shakiest part of my estimate!), that's around 50 million pixels per sheet, somewhere around 6-50MB per sheet. So somewhere between 1/2TB to 4TB for the whole data set (and I could easily be too low or too high by a factor of 2).