Dexter from Dexter's laboratory:
First, if your dad is a lab scientist, you can ask him to let you borrow one of his extra lab coats. Once you have it, you would ask your mom to shrink it in the laundry for you.
After that, you can get a perfect size. (If it's too big, you can just ask your mom to cut the sleeves shorter.)
After your lab coat is done, you will need a pair of purple gloves, a pair of black pants and black shoes. There you go!
Happy Halloween! (oh yeah, don't forget to buy a red wig for Dexter's hair, and also the pair of huge glasses!)
Thanks to Julie for this fun idea!