I found a height comparison chart online. Apparently Manny is about 11 feet tall and Sid would be about 4 feet. Both their heights would be accurate to their species if you believe Sid is a Shasta Sloth. Since they both scale to their respective species heights, we can assume that Scrat would very likely scale to the normal size of a modern squirrel as well. Scrat is not based off any actual prehistoric squirrel, therefore it is safest and most accurate to assume he is the same size as modern, real world squirrels. However, please scale Scrat to Manny and Sid so that we can have a proper height comparison