Combining AVI's

I have read that the next version will have the ability to combine multiple morphs into a single AVI.
Until that version comes out, I will be making numerous morphs that I later plan on combining. My question is: Do all the morphs have to be the exact same size (length & Width) in order to combine them later?
Until that version comes out, I will be making numerous morphs that I later plan on combining. My question is: Do all the morphs have to be the exact same size (length & Width) in order to combine them later?