Can I resize the sd card to smaller?

I have a 64 gig sd card on my Volumio3 device. I use a nas so i don’t need internal space.
64 gig is way too big for backups. Can I shrink my current volumio partition?
Then i can make fast backups. How would I do that
I have a 64 gig sd card in there because I wanted faster boot up as it is a V30 type.

This was discussed in this thread, with a reported solution. It is still recommended to use 8/16 GB cards for Volumio.

Can I shrink my existing 64 gig SD to say 16gb and still work? or will I have to start all over and flash?

Assuming that you use windows or mac. You can make fast backups regardless of the size of the sdcard if you use something like Acronis True Image or Paragon backup and recovery as they only backup the data used. The latter has a free community edition.

or just download a simple and small program like win32diskimager. Totally free.

Shrinking does not make sense: at boot time, Volumio will check free space and resize the cardto its max. size again.