That does not register the CustomSizeAddEvent class....
Nor does your CapabilityProviderEntities class have any event handling methods....
In order to reference your CAPABILITY_SIZE compatible capability you need to attach a CAPABILITY_SIZE compatible capability to the entity. Your SizeCapability class is not your capability, it is merely a static class that holds a reference to the CAPABILITY_SIZE registry entry, similiar to ModBlocks or ModItems. That is, your class names are awful.
Size -> your capability
SizeCapability -> your registry object reference
CapabilityProviderEntities -> a capability provider (that is completely useless)
CustomSizeAddEvent -> your event handler
I'm attempting to add a fire-like overlay to a mob when a certain condition is present. Nothing has crashed, but no overlay appears. The code is adapted for a RenderLivingEvent directly from the #renderEntityStatic method in EntityRendererManager. I don't believe the issue has to do with the texture stitching event, but I could be wrong.