Using spotlight to filter results to to images larger than a particular size works perfectly:
mdfind -0 -onlyin . "kMDItemPixelHeight > 1440 || kMDItemPixelWidth > 1440" | xargs -0 sips -Z 1440
This find images recursively from the current directory with width OR height greater than 1440 pixels and resizes them down to 1440. Files under 1440 get left alone.