As far as I know this is not possible to achieve in a single pivot table. This is because you need to apply two separate aggregations to the same set of number based on a condition (RequestId) It is possible to get what you are looking for using two pivot tables, however I would not recommend it but this is how you would do it.
Create the first pivot table on your base table, add the requestId to the rows and the size to value, this will give you an intermediate table with the sum of size per requestId, you then build a second pivot table, this time using the first as the source pivot table as the source, in this instance you will only add the ‘sum of size’ value and take the average of this. See below for example
Again I would not recommend this approach for anything but the most simple analysis
A better way to do this is to use powerpivot, a separate yet related technology to the pivot tables that you have used. You will need to import the table, I have assumed with the name [Logs] with columns [RequestId] and [Size] you will then need to add a calculation
AvarageSizeOfRequests:=AVERAGEX(SUMMARIZE(Logs;Logs[RequestId];"sumOfSize";CALCULATE(sum(Logs[Size])));[SumOfSize])
This will give you the following result
The first is the strait sum which you already have, the second is the average which will be the same per requestID but will aggregate differently.