How to calculate number of nodes opened by BFS?
In the case of the above diagram, as you mention, there is no constant branching factor. This means that we cannot solve for the number of nodes explored using the formula you give above. 

Instead, we must use knowledge of how BFS works in order to calculate which nodes were visited before Node I was found. In BFS, nodes adjacent to the current node are marked, and then placed in a queue. The next element in the queue is visited (and removed from the queue), and this process repeats. Use this process to figure out which nodes are visited before Node I, and then charge a cost of 2kB for visiting each node.