de ifz

Probably the easiest way to determine the full lenght will be to sequence the product you have and/or clone it, then sequence.

However, just found this paper:
Vaccination of patients with chronic myelogenous leukemia with bcr-abl oncogene breakpoint fusion peptides generates specific immune responses

J. Pinilla-Ibarz, K. Cathcart,T. Korontsvit,S. Soignet,M. Bocchia,J. Caggiano,L. Lai,J. Jimenez,J. Kolitz, andD. A. Scheinberg

Which contains the following paragraph (second one in the paper, immdeiately following the abstract, bolding mine):
"Chronic myeloid leukemia (CML) is a pluripotent stem cell disorder characterized by the presence of the Philadelphia chromosome (Ph1). Ph1 is the result of a translocation of the c-abl oncogene from chromosome 9 to the breakpoint cluster region (bcr), within the bcr gene on chromosome 22, forming a chimeric bcr-abl gene.1-3 The fused genes encode an 8.5-kb chimeric mRNA that is translated to a 210-kd protein.4-6 This p210 bcr-abl protein shows tyrosine kinase activity, is present in the leukemia cells of patients with CML, and is necessary and sufficient for transformation.7 In 95% of patients, the breakpoint in the bcr gene occurs either between bcr exon 2 (b2) and 3 (b3) or between bcr exon 3 (b3) and 4 (b4). Hence, 2 alternative chimeric p210 bcr-abl proteins, comprising either a b3a2 or a b2a2 junction, can result from this fusion gene.8"

Are you doing exactly the same experiment (same species DNA and everything) as the one in the paper?

Have you looked at the specificity of your primers?  Try primer-blast on the NCBI website, if you haven't.

Small products will amplify more efficiently than bigger ones, if there is a potential smaller product in your mix, it will be amplified in preference over bigger ones.

Which stage of the nested PCR is causing the problem?