Monster High dolls come in all shapes and sizes. On one hand, that’s pretty awesome when it comes to realism, but on the other hand… sharing clothes is a pain!
In general, the next size down can borrow clothes from the next size up. The only two molds that seem to be able to share clothes easily is MH Boys and EAH Adult girls. Their lower halves are almost identical, measurements-wise, and their torsos are reasonably close.
Boys fall into the Slimline Male size, and girls fall into the Petite Slimline Girl size (long torso variation), though scaled up/down. See:
Slimline Male Sizing/Scaling Chart
Petite Slimline Girl Sizing/Scaling Chart