What you can do is, get the size in bytes and convert it to KB and then you will have the kb size of your image.
You can get the size by using the BinaryDataSize action in BinaryData.

I have done a little and fast example.
Look at the size of my image

Look at the conversion I did.
I will share my module with the example. Have a look and let me know if it helps you.
What I did was just use the action to get the size, and then convert it to KB

Test on this link to :D
https://marcio-carvalho4.outsystemscloud.com/converttoKB/VsalidateImage?
Kind Regards,
Márcio