Will a BLOB Column always consume the defined size even if less data is inserted?

I need to store images in a DB2 Blob field. The average image size is about 200KB but in rare cases there will be images with 2-4MB. I don't want to reject these images so I guess I'd define a BLOB(5M). Is this okay to do or will this Blob always consume the 5MB even if most of it is unused?

What is the common way to deal with the Blob size if it is hard to find an average?

The blob will only use as much space as necessary. There is no overhead in defining a large maximum (think of it as a "constraint" rather than a physical thing)