The answer depends which product you are referring to.
For iris360 it's about 3Mb for an HDR On image. For iSTAR it's about 4Mb
BTW - the size of the pano in MB isn't dependant on ISO (although noise in the image will have a slight impact) it's the resolution of the image in pixels that has the biggest effect and if you are processing in Immersive Studio the option to enable or disable compression also affects file size in Mb.