2 1/2in diameter is pretty damm big! They're actually around 2 inches diam.
The Coast Guard approved marine flares that propel a parachute flare into the sky 1000 or more feet, ARE REQUIRED for any boats venturing into the Pacific O. off our Woorsheengton-state coast.
"Expired" marine" flares I have found are often for sale at gun shows I attend. And whenever I have shot those flares i make certain to do so on July 4th or New Year's Eve only! Lest anyone who sees them takes them for REAL distress signals.
If you ever visit WA state, make it a point to go into a boating shop and ask to see their "flares" for sale the Coast Guard recommends and approves. Probably answer all your Q's pretty fast!