You can resize in a couple of ways. If you already have a vector/shapefile or region of interest defined for the area of interest, you can use ENVI's Resize Data tool, select spatial subset, and select the vector and/or ROI. However, this type of resizing always returns a rectangular image defined by the min/max extents of the input vector or ROI. To subset an image with irregular borders, you will need to make a mask from the vector/roi and apply this to the image. The result is still a rectangular image, but areas outside of the vector/roi borders can be set to a background value such as 0, -9999, NaN, or other value.