Tallness is primarily the result of good nutrition and good health whilst still a child. A few minor diseases or regular bouts of sickness will stunt growth whilst the body redirects resources to fight off infections.
Good nutrition requires a higher proportion of protein and fat than is in vogue at the moment, and plenty of (but not excessive) calories. Environment plays a lesser third part since its no good eating well if all you consume is being spent to keep you warm in a draughty cold house rather than building new bone and tissue.
Scandinavians win on all three counts and have done historically.
Low population density and (until recently) more rural life meant that disease was less rife. The last couple of generations have been more urban, but this has been balanced by better health care, especially with the advent of universal immunisations.
Food wise Scandinavians have historically had more access to fish, meat and dairy products in their diets, with less subsistence on grain. Again the lower population density and rural distribution meant more food per person, with slightly less impact from the occasional famines incurred by crop failure or war.
Environment-wise, Scandinavians are blessed with a stable climate not prone to severe weather effects, no significant seismic zones, and few poisonous creatures. The winters, although long, historically prevented much population movement (which helped prevent epidemics) and encouraged the building of very good houses. The dryness of the climate (at least on the eastern side of the Fj?llen/Kj?len reduced dampness related sicknesses and physical heat loss.The only downside were the mosquitoes which used to be a vector for mild malaria, but again, Scandinavia was less affected than the rest of Europe, being cooler and having less marshes.
So all in all, Scandinavians with their excellent healthcare, attention to fitness, high standard of living, good housing and pleasant climate are all conducive to permitting children to
more likely reach their full potential growth.