In terms of your question, Giant-Man at his tallest would be far taller than Groot has ever been seen to grow. However, when Giant-Man was at that size, he found it difficult to do much of anything useful and preferred to stay about 18-30 feet tall, giving him strength and some degree of mobility.
In his first appearances (in Tales to Astonish) Groot was seen to be over forty feet tall. Note the buildings he stands next to and the truck he is entangling on the ground beneath him.
It's noted the larger he was, the slower and stiffer he was, possibly due to his increasing mass and the square-cube law that Marvel rarely applied later in its comic depictions.
When we meet him again in his Guardians of the Galaxy appearances, he is much more slender and averages about 18-24 feet during his time with them. This appears to be an ideal fighting weight, making him strong and durable enough to be a threat to the Phalanx warriors, for example.
His most recent depictions (particularly in the movies and the animations) at 8-15 feet tall.