What size should gas grill burner flames be

Sounds to me like a simple case of a blocked jet. Depending on your competencies with gas appliances either get someone to service it or you can do it yourself.

For the latter, locate and remove the jet for that burner, normally located closer to the know than the burner itself. Using a proper jet cleaning wire clean the jet by pushing the wire into the jet from the outlet end, so that any crud is forced backwards, rather than through the jet.

While you have it apart, check the inside of the burner pipe and remove any scale that has built up, which could come loose and reclog your jet.

Of course if you can get hold of a replacement jet you may find it worthwhile simply replacing the clogged one.

You would first turn off the gas at the meter. Then, if this pipe extending from the wall is indeed a gas line, you would take off the cap, clean the threads well, install suitable thread sealer made specifically for gas lines and install an appropriate gas shutoff valve. Install the grill gas connection down flow from this new valve using adapter fittings if necessary.

Then in case you ever need to disconnect the grill for winter, cleaning, replacement, moving or whatever else you can shut off this added valve without having to turn off all the gas in the house.

If you ever do disconnect the grill from the valve it would be necessary to cap off the open valve end for safety sake --- just in case someone comes along and accidentally turns on the valve.