Usually, the collision radius of an NPC is stored in - /gameserver/data/stats/npc/NUMBER-NUMBER.xml
where NUMBER-NUMBER is the range of NPC ID (example 0001-1000 means the stats of NPCs with ID between 1 and 1000)
Alternatively, you can also play around with your server pack's NPC stats parser. There you should be able to add an exception for the said NPC (by ID), and in that exception, you will tell the server to parse a custom setting for Collision_Radius for that specific NPC or a range of NPCs by ID.
Example:
if (npc.getID() >= 1001 && npc.getID() <= 1999)
{
npc.setCollisionRadius(0);
}