I have a similarly sized one, and it doesn't use much propane. 5-10 psi through a 0.03" MIG tip (this burner is probably similar). uses about 1/4 of a 5 gallon tank after 6 hours continuous use. melted well over 4 kg of silver in a small crucible in that time. (I like to save up silver crystals until I have a sizable amount to melt).
that burner pipe is either 1/2" or 3/4" (it's a name, not the actual size. actual OD on 1/2" pipe is about 1-1/4"), so I would estimate the OD of the furnace to be about 12" (30 cm, for those that are part of the vast majority of the planet), then it looks like they're 2" walls for an ID of 8". that can accommodate a fairly large crucible... about #8 = 11.5 kg silver.
of course, I'd ask the seller for the exact dimensions before buying.
it's best to first decide how much you want to melt at a time, then get a crucible that can hold a bit more than that, then get a furnace that is a bit larger than that (crucible outer dimensions should be 3/4 furnace inner dimensions for maximum efficiency).
