If this is specified, all the animation data will be written to a .ani file, and when sequences are needed, the data will be loaded on demand. This is helpful for streaming animation data of Cinematic Physics. The "size in K" is the block size the animations will be grouped into. Sizes between 4K and 64K are fine, though 16K or 32K has better memory fragmentation characteristics.
nostall option will make the first section not demand load but by included in the .mdl file.
$animblocksize (size in K) ["nostall"]
For example (In Left 4 Dead 2 modelsrc qc, "props_destruction/general_dest_plaster_set.mdl"):
$animblocksize 32 nostall highres