What size gas line do I need to run to my garage
Step one would be to check with your local inspection authority. Gas work by non-licensed professionals is prohibited in many jurisdictions.

If it's not prohibited, they may offer advice on material and installation requirements. You can also talk to the employees at a local gas supply store.

A good option for your project would be a direct burial flexible gas line, a.k.a. poly gas pipe. Such a line could be installed so that you have no buried fittings.

Whatever you bury, make sure you place a layer of AT LEAST 6 inches of sand over the line. You should also put a marker tape over the line. The marker tape and the sand serve as indicator to future diggers that something is buried below. Pea gravel below the line is a good idea so that any water that seeps in leaches away quickly and does not stay in contact with the line.

For an example of how to use the sand and marker tape, see the segment of this video from Ask This Old House where they install a blue stone patio. And here's a crude diagram: