I don't think 100 pages for a refusal to assess appeal is unreasonable, bearing in mind the very low threshold that has to be satisfied. It shouldn't normally be necessary to file much more than something like paediatric, EP and school reports.
There will be core documents that don't count against either side's totals, including the appeal, response, tribunal orders, EHCP/statement, and relevant annexes. Therefore it's in parents' interests to get as much as possible by way of past reports annexed to the EHCP. It will be possible to apply for permission to file documents that take the page numbers over the limit, and in complex cases that need a lot of reports I wouldn't have thought there would be any problem. What they're mainly trying to avoid is the situation where there are hundreds of pages of old reports that really aren't relevant, or endless correspondence, or the type of situation where someone's done a DPA search of school records and files the lot whether they're relevant or not. However, I guess experts are going to have to cut down the length of their reports, and I don't think that's necessarily a bad thing.