How to selectively filter BLAST results for an endogenous retroviral LTR to retrieve members of the same ERV family?
I don't think there is a definite answer to this problem, but here are a few things that might help you, apart from a simple length filter for the hits:


Filter for regions: since LTRs are repeats you might get multiple BLAST results from the same repeat region/locus (especially if your input sequence is smaller than said region). You probably want to keep only one hit per region
Check for a conserved (sub)sequence: hopefully your specific LTR sequence has a 'core' that needs to be present, so that it can be considered part of your familiy. You only want to keep BLAST hits that have an absolute match for this 'core'.