How do i save logs with selected attributes only?

Presuming you have the capture files, you can use tshark to output the fields you require. The trick is knowing the names of the fields, I usually open the capture in Wireshark, select the fields in the packet tree and the field name will be displayed in the status bar at the bottom.

In your case you are asking for the time and size, presumably of each packet, and the number, do you mean frame number?

The command line to produce the time, size and number of each packet is:

tshark -r yourcapturefile -T fields -e frame.time -e frame.len -e frame.number

Read the tshark man page for more info on the -T and -e parameters.