I've been using grobags since dd1 was a baby 9 years ago! At the time they weren't available in the UK, so was begging them off friends in Austria and Germany.
Countries that have been using them for years don't make a big thing about tog rating, and I never knew about it until grobags took off here. I've just used common sense: thick padded ones for winter, thinner ones when it's warmer, with varying combinations of long/shortsleeved vest and/or babygro underneath as appropriate.
It's not an exact science, so I wouldn't worry too much. Thin ones probably get more use than v. thick ones, as you can just put more layers on underneath. The same with the size -- unless the neck opening is so huge the baby could slide down inside it, I wouldn't worry. Some of ours are ridiculously long, but it's the fit at the top that matters.