Rather than imagine replacing Yoda with another small character wouldn't it be easier to replace him with a character witha large silhouette. For example a different small character could involve masking Yoda out, then replacing the background and adding the new character in. Or just carefully matching the head angles and masking and replacing just the head (and recolouring the hands or something). Whereas if you had a human sized character with a wide cape you could just cover over Yoda in most of his scenes. This might involve rescaling certain shots and backgrounds do to the height change, but overall I think it would be less work.