first set up at least some logging, like to memory or disk (in case you have FortiGate equipped with disk), as close to intended logging filter (config log memory filter) as possible.
Then keep it logging for some time.
And then go CLI to see something like in my example below:
# diag test app miglogd 4
info for vdom: root
traffic: logs=1160137 len=627074265, Sun=89458 Mon=132174 Tue=225162 Wed=239396 Thu=145690 Fri=153707 Sat=60834
event: logs=93493 len=29698750, Sun=1173 Mon=1545 Tue=1372 Wed=1403 Thu=825 Fri=1778 Sat=1107
traffic: logs=1160137 len=627074265, Sun=89458 Mon=132174 Tue=225162 Wed=239396 Thu=145690 Fri=153707 Sat=60834 compressed=37039926
event: logs=82559 len=24792447, Sun=885 Mon=1257 Tue=1084 Wed=1115 Thu=651 Fri=1490 Sat=819 compressed=5176706
I know those are not actual log sizes but amounts of log messages, but they might give some insight to amounts you face.
In case you have a log disk .. show log disk usage
# diag test app miglogd 16
VDOM log disk usage:
Once you have some external log collector (could be simple Syslog on some external machine), then you can see usage in GUI
Tom xSilver, planet Earth, over and out!