Hello,
You can skip the "size" option if You are not interested in packet payload, only in IP/TCP/UDP header.
Otherwise - always specify "size" option _UNLESS_ all of Your interesting packets fit into default capture size 96 bytes, see below
aarseniev@mx480> monitor traffic interface fxp0 no-resolve
verbose output suppressed, use <detail> or <extensive> for full protocol decode
Address resolution is OFF.
Listening on fxp0, capture size 96 bytes
HTH
Thanks
Alex