I suggest you start at the beginning. First you need to collect the families that you are interested in. Then you need to query them for location on a local drive, if they were not saved then you would have to temporarily write such family to a folder somewhere on a drive. Then you can extract file size, or what actually is called its length, since that will give you the file size in bytes which you then have to convert to kB or MB. This is relatively complex task for a Dynamo user, since there is barely any nodes out of the box that can do any of the things that I just mentioned. You will need to pick up some Python or C# skills…good luck!