sadly
you may have to buy a bike sight unseen
which is often the case for riders at either end of the fit spectrum

i am 6'5" and most
but not all
xl bikes fit me ok
but many are still a wee bit too small

you can get a really good idea of the fit of a bike
by studying the published geometry
and comparing to a bike with a known fit
even if the known fit is a poor one
you can estimate how much of a difference you need
and see if any of the available bikes are close

be very careful
however
if you plan to buy a bike that is too small
and mod it to fit
as there is a limit to how big a frame size discrepancy
you can overcome with this strategy