You can use different size drives in the NAS. Just keep in mind that if yo uwant to setup a RAID the size of the array will be restricted to the smallest drive in the array, which in this case is 2TB. It's generally frowned upon to use different drive types, let alone sizes for RAID, but it can be done if you don't mind sacrificing the extra space on your larger drive to do it.
Another option is to use the HDDs as JBOD so they could be two different storage locations.
Hope this helps and feel free to ask any questions you may have.
Hi I need to replace a dead seagate central nas drive and have been looking at the wd my cloud ex2. I have removed my 4tb hdd from the seagate so was thinking of purchasing the wd without any drives in it (a 2 bay) and putting my 4tb in it. My question is do I need to fill both drives with the same size hdd or can i combine 4tb and 2tb for instance or indeed only use one drive,