In reply to ksjs: Not fixed... but not all sizes are freely available.. the tall thin ones (47kg) are too big and heavy for domestic retail and there are some sizes not taht common.. I think I have 19kg which is a sub waist, stumpy one that not many places stock
Start by calling Calor themselves as they do direct sales via their local representatives and see what their price is... then decide if a few quid saving is worth it for the agro and petrol to go to a depot somewhere unless it does happen to be a size you can pick up easily somewhere convenient.