It's not just in "Service Manuals, Requests, Repair Tips", test instrument related questions can be found in other sub-forums as well.
Also, I can't see how "Service Manuals, Requests, Repair Tips" would be a proper place for asking test instrument related questions, the title alone suggests that it's more concerns with repairs and the actual item to repair.
My suggestion to put test equipment related questions in a separate sub-forum is simply because testing properly is an art in itself, with a lot of errors made even by experienced engineers. This is even more true for selecting a new piece of test equipment, which is difficult if you don't know the market.
As to the forum size, I think it's more of a problem of organization, i.e. too few categories for the many topics that are discussed on EDA.