Concrete pillar foundation
The spacing of the sonotubes is determined by your foundation design, specifically how big the beams are going to be. The tradeoff is that larger beams cost more, but the spacing on the sonotubes is greater. The required size of the tubes depends on your soil conditions, and the load on each support. You need to do some soil sampling before you can size the tubes, unless you go with minimum sizing from the local building code.
Required depth of the footings is typically based on local building code, check with your local building inspector. Often the building code will dictate the sizing and connection details for all structural elements, best to check with the building inspector. If there is no building inspector, or you are not required to pull a permit, it is generally safe to design to the building code for your state or province.