You are right, the length certainly shouldn't make any difference for most viruses.
Eicar, however, is a special case. This special restriction has been incorporated because some true viruses attached (or prepended, or somehow used) the Eicar test string to make them look like "Eicar - not a virus", even though they were really dangerous. To avoid these misdetection, Eicar detection has been restricted as Vlk posted.