The building I used the trench for was a 24 x 48 story and a half timber frame. As for size limit I am not sure what you mean but I would imagine most residential/ light construction will fit. Compacted gravel has very high load bearing capacity so the real limit is the soil which in turn supports the gravel. Soils that will wash away or into the trench can not be used. You can also check the JLC Forums, rubble trenches have been discussed there as well.