I'm going to try these for my two sister-in-laws who are each going to give me their cat once I move.... they'll be a sort of replacement
Looks simple enough and I think the size will depend a lot on what material you use. I don't know how well they will hold up when actually used as a cat toy; I mean you could easily stuff them with catnip (is that the word?) but since you're sewing the parts together, they might not withstand the stress of being chewed, puled and clawed at.
but if it's just a toy for a child or something that you're looking for, this might be something for you