If I can only offer a small amount of traffic e.g. 20 GB per month for my middle relay would you recommend to set a monthly limit or a daily limit?
Setting the traffic limit to 20 GB would mean the relay is running one or two days and hibernating most time of the month. Setting it to 660 MB per day would make it run every day for approx. 2 hours and hibernating the rest of the day.
Would you recommend to set the monthly limit and let TOR do the balancing or give a small daily portion?
In general it is better to have a maximum bandwidth for the users. Tor needs some time until it reaches the maximum bandwidth and this will probably take longer than a day. So it is better to set a monthly limit.