Now, that will depend on what you are going to do with the model. The actual process to determine loads for building the structure are a bit more complicated than this but, to start off, pick a suitable g-load and multiply the model weight by that factor and that will be a reasonable point from which to start the thinking process.
The span isn't a determining consideration for the load but is for the structural design to accommodate the load .. for interest, what load did you come up with ?