could any one tell me if the relevant codes allow for the following situation:

A psv needs to be serviced while the unit is still operating. Therefore, isolation valves are installed and a manual bypass line is used. An operator stands by on radio and opens the manual valve in the case of an emergency.

If this is allowed, then are there any suggestions on how to size the bypass line and valve?

If the philosophy is to have a PRV serviced when the plant is on-line then a standby PRV must be provided. You take a risk operating in an isolated mode. However, operating plants frequently have these scenario - for which the best tool is to carry out a risk assessment and present it to the management. Let the risk level decide the decision. This is what we call a probabilistic approach as against a deterministic approach .

Some valuable comments above.

What is the sizing basis for the relief valve.  IE. Is the vessel in the fire zone? What is the valve protecting against - what is documented in the sizing case?

If you decided to proceed you must quantify the risks. I would suggest using risk assessment. 

For your info.  I have personnaly installed a valve below a relief valve.  But we found the risk unacceptable to change the valve while the plant is running but having the valve allowed a much shorter shutdown while the valve was changed and the plant was shutdown.  We used a castle key system to ensure isolation of the energy sources.

I've also heard of on-line testing of rv valves, have you considered this ? i think its called trevi testing

Or are you certain that the servicing is required?what the driver?, have you a history of RV's losing their set pressure for this service or have the valves been reliable, could you extent the test interval.

Another option I've seen used is a BD by-pass system for RV while they are being serviced.

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