I'm running into similar problems, although with slightly different results. I've inserted the jQuery into a customized NewForm.aspx on a list called "Objectives". Here's my code:
multiSelectColumn: "Collaboration Channel(s)"
After filling out the form fields, and pressing okay, I get returned to the newitem page on the Objectives list:
I then correctly get redirected to the redirect url:
I fill out the form on the Next Steps list, and press OK, which gets this url and a blank Next Steps form:
I'm also not able to get the SPSetMultiSelectSizes to work at all on the NewForm.aspx on the Objectives list, unfortunately.
I'm hoping this feedback helps you out a bit in figuring out what's going on. Let me know if I can get you any sort of debugging feedback as well!