The top margin of a specified memo is 2 inch.The default margin format on your word processor is probably two-inch margins at the top and bottom and one-inch margins on the left and right. These settings are okay for the traditional legal memo, and you can use them unless those you work for tell you to change them.
Your line-spacing default will be single-spacing, but most formal memos are double-spaced. Double-spacing leaves room for the reader to write notes between lines, but it also uses up twice as much paper or takes twice as long to scroll through on the screen. It also makes it difficult to skim the document. Still, double-spacing is a tradition in legal writing.