this is the answer provided by Help Support and works:
Thanks for following up! It looks like you already resized the Droplet to the 8 GB size, which is why you’re unable to resize it to the “same” size right now. What you would need to do is resize the Droplet to its original size, and then expand the Droplet to the 8 GB size again. The difference is resizing only changes CPU and memory, while expanding changes CPU, memory, and disk space. Also, like the names hint, a Droplet can be resized up or down, but it can only be expanded to a larger size, since shrinking a disk has a risk of filesystem corruption and thus isn’t supported.
Let us know if you have any other questions along the way and we’ll be happy to help!