Syntax
set blobLength = aBlob.blobLength(); 
SemanticsReturns the length of the blob as Integer.
SubstitutablesblobLengthCan be any variable or object attribute having the type Integer. 
aBlobCan be any variable or object attribute having the type Blob.
Examples
set size = aBlob.blobLength();

Example File (Builder project E2E Action Language/BaseTypes):

<your example path>\E2E Action Language\BaseTypes\uml\blob.xml