I'm going to build a Siporex enclosure with a tile roof around the stove, so the stove is actually built in a large cube. What diameter and height must be the steel chimney pipe and do I need to insulate it?

The smoke pipe should be at least 2- 3 meters high to get a good draft. For the diameter the ideal dimension is 180-200mm, for lengths over 4m a pipe in 150mm should be sufficient, depending on weather, wood and other factors. A single wall pipe is sufficient, double wall pipes are only relevant for close combustibles or roof penetrations.