This is a rough, high-level way to estimate - here's how I go about it:
I get the total model size by going to the gear and double clicking on the cell count that shows up next to the "Model Settings" header (you can also get the model size by going to "Manage Models" and it will be displayed by the model name)
I get the total cell count either from the same location (rounded) or by adding up the cell counts from each module
Divide the model size by the number of model cells to get a size per cell
To calculate the size of a given module, multiply the cell count from that module times the model size per cell from above
Again - this is rough and does leave out the size of lists (which may or may not be material, depending on how your model is set up) and possibly other factors as well. But it's a good way to get an estimate.
After my research on size calculation, I have found the below point from community itself. But I don't have the exact link currently. However, this calculation helped me for optimizing my model.
Model size in Anaplan is based on the number of cells in the model and each cells takes up approximately 10 bytes of space. This converts to each cell taking up 0.000000009313 GB so the size of a 1 billion cell model in Anaplan could be estimated as 1,000,000,000 cells x 0.000000009313 GB per cell or 9.3 GB.