The supply size field is an upper-bound, so you should be OK to put in a small number like 1 or 0.5 MW. The model uses the supply parameters mainly to price the supply, so given:
- Capacity (actually max. capacity)
- Fixed cost
The model should try to build a network so that the difference between revenues and costs is maximised, subject to the requirement that supply not exceed capacity. Revenues are determined by the building's annual demand * tariff price, and costs are determined by pipe costs + supply costs. Pipe costs are in turn determined by peak demand, and supply costs by peak demand (through cost/kW) and annual demand (through cost/kWh).