Simply do subset each time without selecting an AOI or Box (to keep the same image size), but select each time one band. Do this for as many bands as you want to separate.
I think you would like to separate stacked bands in to single bands. if it is so..the process is very simple. you may used the same process as used for stacking layers. There you can select only one band at a time. Similar process need to be repeated 7 times if you have stacked seven bands. Hope this will be helpful. You are most welcome for further help.