Huge Trees are generated only in Mystcraft Mod. While Vanilla Minecraft generates a normal Oak Tree, the Mystcraft Mod generates a Huge Oak Tree.
Huge Trees vary to places to places. The biggest Tree can be found at Mystcraft is on Jungle Biome... Think About it when Vanilla Minecraft generates an enormous Jungle Tree, the Huge Junge Tree is the biggest.
But don't forget and Fir Trees, whose size is big enough, can pass in height the Jungle Trees, so do and Huge Fir Trees!!
Biggest tree ever: 100 blocks!!!