WASHINGTON — The still-secret report on Russian interference in the 2016 election submitted by the special counsel, Robert S. Mueller III, last week was more than 300 pages long, according to the Justice Department, a length that raises new questions about Attorney General William P. Barr’s four-page summary. Mr. Barr wrote to Congress on Sunday offering what he called the “principal conclusions” of the report — including that Mr. Mueller had not found that the Trump campaign had taken part in a conspiracy to undermine the election. But he had notably declined to publicly disclose its length.