The operative term for raster (Image) files should always be PPI (Pixels per Inch), and never DPI.
At a file resolution of 100 PPI, an uncompressed RGB raster file will be slightly over 4MB in size per square foot of area in the file. For image files, 100 PPI is normally sufficient resolution to print at production print resolutions (typically between 300 - 720 DPI). This is assuming that the file is at print size, and no scaling is occurring in the RIP/ printer driver.