Nominal Length: Average distance between the paired reads in base pair. This length can be determined by measuring the size distribution of the DNA fragments used for constructing the library.
The definition is vague, but my understanding is this is intended to mean the approximate insert size estimated during the library construction. It is "nominal" because people frequently fill this field before mapping and in that case, we do not really know the exact distribution.
I could be wrong. I do not work with SRA often.