Have worn them for both work & just chill'n & never had it come untucked or expose any thing,I hope this answers your question.
I don't know in that i ordered large shirts. I can say that in Wolverine and Carhart T-shirts I wear a medium. With these less expensive T's I always order a size up. The length of the Large unwashed Gildan T from the top of the shoulder seam to the bottom of the bottom hem is 281/2 inches.
Do not know about M.
L is 26.75 " . Wish this is helpful.