One Charles de Fleury catalogued pieces of the true cross in the 1800s and found them to sum to 2% of a plausibly sized cross, not the oft-repeated claim that they add up to a large ship, or a nice house: https://en.wikipedia.org/wiki/True_Cross#Dispersal_of_relics paragraph 6. While perhaps biased, it does seem plausible that a cataloguing of such claimed relics today would add up to a similar total volume.