General answer
to sum entire Log Dir (on Mgmt/Log-Server):
du -sh $FWDIR/log/
a specific day (Aug-26th - 2019)
du -sh $FWDIR/log/2019-08-26*.log
(Add ~6% if you just count the .log, as it's simpler)
I'd calculate your log dir size of one day & choose storage accordingly.
for per GW on R77.x
Ed's suggestion or probably simpler:
Open SmartviewTracker > Pick a closed log-file (a representative avg one) > Filter by your chosen GW's origin > Count no. of logs.
Calculate the time-frame of this log-file: between the previous one's closing time & this one's closing time.
Extrapolate this GW's no. of logs to a full day and so on...
for per GW on R80.x onwards
You'll be able to easily use the cpstat mg/ls -f log_server/logging on your Mgmt/LS, as suggested already.
for a GW: cpstat fw -f log_connection