I just removed my seatpost and it is a 26.4 mm seatpostI dont know the size of the seat tube as I dont have anything to measure itWhat size derailleur would fit? 28.6 or 31.8?
That depends on the tubing thickness on the outsideSteel most likely 28.6 and Aluminium the latter (but that sounds a bit too thick) These days they are often universal but not alwaysThe best bet is to measure it either with a piece of sturdy wire and then a ruler or even easier get a bit of cardboard and cut a suitable slot in it, trim gently until it slips over the tube them measure it.
Everyone has a different opinion on this it seemsI chose to be safe and give it a chanceI got it today as a health care workerI think if anyone in your family has any underlying health conditions or are immune compromised I would get it for sureIt seems to be hitting children and teens the mostThat's just my opinion thoughI would do it.