Another vote for Uniqlo - tend to be on the long side. Tall ranges. La Redoute Taillissime (plus size range) is good for length if you are size 14+. The length of their tops increases with each size up. But the quality and colours can be a bit dodgy - choose carefully. I like them for their cardigans and some vest tops are good.
If you are size 14+, shops that cater specifically for the larger lady tend to have lots of long tops but it is finding the few decent plain ones amongst the piles of vile prints and gathered jersey tents.