This is something we've never recorded in NavigationTiming, afaik, despite it being available in the corresponding PerformanceEntry. As @stjn pointed out, it would be interesting to see how it correlates to survey responses. Eg. this might inform community decisions to split up large articles into smaller ones.
It would probably be useful to also record the total transferSize of all resources from ResourceTiming (maybe totals per category? images/scripts/etc), in addition to the main document's.