The book for the wall oven I purchased said to use 1/2 in. id. Flex line. The people that installed it purchased 1/4 to 3/8 in id. Line. The propane line is 1/4 to 3/8 in id copper line. Should they have used the 1/2 in. id. Flex line. The flame doesn't seem to burn right. I have adjusted it but it still burns yellow. They said they turned the piece for propane instead of natural gas. I have 1/2 in id. Line for the cooktop and it burns blue.

If you have too high pressure on the gas leaving the regulator on the oven you will have a yellow flame because it has too much gas for the amount of oxygen. You did not say what kind of gas you actually have the valve set up for. When the gas is at a high pressure coming from the tank it needs a smaller line like 3/8" id/1/2" od line. Once it goes through a regulator you need 1/2" id line, especially when it's flex as the flex adds a tremendous amount of friction just like 2 dozen fittings. What you need to do is to hook a gauge up to the burner to see how many inches of water column pressure you have. Natural gas should be at 3.5" of water column and liquid propane (LP) should be at 11"; however, the oven manufacturer may have a different pressure they want at the test point which is at a plug on the burner, on the gas valve, and on the manifold before the orifices. Look on the nameplate and get the BTU rating of this oven and use this chart for guidance: http://dl.dropbox.com/u/8010163/Pipe_sizing_chart.pdf
Never have a yellow flame because it is carbon monoxide. Once the pressure is correct adjust the air open until the blue flame just starts to lift off the burner. You want the flame to sit right on the burner so that if you add any more air it will begin to lift off the burner. Do this for the stovetop burners, also. This will give you the hottest flame with no carbon monoxide. You should install a CO detector, and you should make sure all gas lines are sealed with pipe dope. I hope this helps.

That and propane burns at a lower temp, less BTUs. The smaller line does not reduce the pressure but it does affect the volume of propane and should be corrected

One note: if you will post the manufacturer and the model number we can tell you more. When one has to change the gas valve for a different type of gas each manufacturer has their own procedure for doing this. There is a change in spring tension for the diaphragm in the gas valve which is often done by removing a cap and turning it over then putting back in, and changing the gas orifice(s). LP orifices are smaller (about half the diameter) than natural gas orifices.