Derivative files are typically created before the ingest process is started. In most cases, each collection has a custom script (in the DMIC's space on Gavotte) for derivative creation.
(See the current scripts on Gavotte for the current practice.)
Image derivatives can be created using ImageMagick command-line utilities (convert or mogrify). The best example script to run these utilities is in the Hohenberger CVS, hohenberger\data\load_utils\convert_hoh.pl.
Our "typical" sizes for derivative images are: