You should probably start by using a hex editor that has support for editing files with semantics, like 010 editor. There you can just select the size field and enter the file size as number, without converting to hex yourself.
12.8mb is not the accurate size, you will need the size in bytes.
the *FID chunks are documented on the wiki and follow the MD21 chunk in legion m2s. They contain ids of the .skin, .anim and .phys files for the model.