You can't use the secureFrame in the snippet, since the snippets are processed in a secure environment, you an use self instead the secureFrame since it's the one called the Execute.
And to run the snippet codes during a script event, you also can't Hook or set the script, you can only do it like
Lua Code:
secureFrame = secureFrame or CreateFrame('frame', nil, UIParent, 'SecureHandlerAttributeTemplate')
SecureHandlerWrapScript(MultiBarRight, "OnShow", secureFrame, [[ -- do the job ]])
Unfortunately, the OnSizeChanged isn't a script type supported by the wrap script, so you can't do the job through the secure snippets.
The MultiBarRight's scale will be reset in
MultiActionBar_Update, it's called under several conditions. I can't say it's a good choice to disable it, so you may keep scale it out of combat, or just change the uiparent's scale, or just replace it with an action bar mod.