When measuring for a blanket or sheet, as shown in the diagram, measure from the center of the chest, around the side of the mini to the center of the tail.  You should also give us the height of the mini at the withers.  Rule of thumb is if your mini is 30 inches tall, the approximate size will be 10 inches more than their height so the size would be a 40 inch blanket or sheet.  This is only approximate, so when measuring you will know you are measuring correctly.