From Purdue OWL's page on citing a memo: "The format of a memo follows the general guidelines of business writing. A memo is usually a page or two long, should be single spaced and left justified. Instead of using indentations to show new paragraphs, skip a line between sentences. Business materials should be concise and easy to read.

Purdue’s OWL recommends breaking the memo up according to the following ratio: The heading should be one-eighth of the memo, the opening description of the audience and purpose should be one-quarter of the length, the discussion of the subject one-half of the length, and the closing segment, summary and note about attachments the remaining one-eighth of the memo.