Maps with the same DPI have a different MB of, a fact that I think is link to the complexity of the project. As an example a map with a HD raster will be with higher MB relatively to a similar map without the raster, both maps with the same DPI. My question is: Can I choose the MB of the export image like the DPI?
It is possible to precisely calculate the exported file size for some non-compressed formats (e.g. TIF without any compression, BMP, etc.; this is a handy file-size calculator). It depends on dimensions and color depth. Some header information might slightly increase the total file size.
However, to keep file size smaller, it is good to wisely use some compression (in case of loss-less some one prefers to say compaction). You can save a lot of disk space, but you cannot estimate precisely the the file size of the exported map. The results might vary o lot, depending on the map complexity, selected format and settings.