Thanks, everyone! That sort of helps narrow it down. I should probably aim for the upper end (ie 7), given my wide and fussy feet.
Inge, Zappos.com refuses to allow access from India That does sound like the most sensible way to look, with actual measurements.
Of all things, I'm contemplating buying boots that can stand up to a downpour --- which is a pretty expensive purchase as it is for me, with both returns and shipping adding to costs. But I'm getting the impression I'll have to either pony up for a holiday abroad or continue to do without if I don't take a chance on an online purchase, since this is simply not something one sees much around here!