Not to be confused with IC2 Rubber Trees, the RedPower Rubber Tree is a very large tree only found in jungles.
Rubber trees take an incredibly long time to grow using normal methods (a matter of real world hours), as Bone Meal will not work on them.
Be sure to give a sapling enough space to grow, as the tree is huge and has a trunk size of 3x3, when grown. It may be necessary to place a Dimensional Anchor in order to keep the chunk loaded constantly. Player-grown Rubber Trees tend to be slightly smaller than naturally grown ones.
The required space appears to be a height of 25 blocks and a width of 13.