Hex binary does not appear to have a bounded length. All the schema says about it is that it is finite.
Why does the collada schema specify this:
The only place this is used is for images. Could this perhaps be for pre-generated mipmaps? The images would be whitespace-delimited I guess.
The "ListOfHexBinary" type is only used when you want to embed the image data in the COLLADA document. This could be a single image or a whole mipmap chain. Currently I know of no implementations that support this feature.
The "ListOfHexBinary" allows for flexibility within implementations. One implementation might want to whitespace delimit the different images, as you suggested. Another might want to whitespace delimit each row of an image. That is up to the implementation. As with any other "binary" data there are many things left to the implementations: endianness, precision, etc.