I agree. I have seen customers use a variety of different settings here.. some as high as 100MB, some as low as 10MB. There are a lot of variables in play here.. the reason that most customers introduce the limit is to decrease the file sever latency (since the end user file open is delayed by the time it takes for the AV scan to complete).
The quicker the total AV scan time, the lower the latency. There's many variables that affect this - the time it takes to read the file from disk, the time it takes to send across the network and the time it takes to scan the actual file. The read from disk, read across network are items related to general performance - so things like storage configuration/setup, hw, AV protocol used, current workload all play into that. The time it takes to actually process the file (scanning), depend more on the HW characteristics of the AV scanner, its current workload/overhead, the file type (zip files typically take longer due to extraction) and the AV algorithm itself. There's just too many variables in that equation to say that "x MB" is your perfect answer - the customer is going to have to experiment to find the optimal size for their use case/environment.