Look up a basic (video or with screenshots) tutorial about passing variables from the Blueprint to the Animation Blueprint (you basically just get the owner in the AnimBP, cast to your class, then drag off the casted BP class reference to get at your sizes). Here’s an explanation though:
So you’d have e.g., HeadScale vector in your BP_Character as an editable variable.
In the AnimBP for your mesh in BP_Character, you’ll do the Get Owner node, then Cast to BP_Character. Drag off the reference from the Cast node and type “HeadScale” and your variable should appear.
Then you just need to make a corresponding variable in the AnimBP, and do a set node that sets it to the value from your BP_Character. You’ll then use the AnimBP’s variable in your animation graph.
Then do this for each bone you want to be able to scale (so probably most or all of them, though you can do things like set the scale of all the spine nodes at once etc.), with their own variable in the BP_Character and in the AnimBP, and set each one after you get the BP_Character reference from Get Owner -> Cast to BP_Character nodes.
And of course you already have it set up but you then just put some Modify Bone nodes (one for each bone you want to do) and plug in the variable from your AnimBP for the scale, and set to Replace, and fill in the bone name.
Hope that makes sense.