The Office of Public Sector Information has issued guidance on ways in which official publications should be made available in formats that enable disabled users to access them
The RNIB's 'See It Right' guidelines give more detail on font size. RNIB Corporate Publishing can also provide advice on designing and producing large print
The Disability Rights Commission produced a guide to easyread, which is now available from the Equality and Human Rights Commission. It has a number of examples that explain the format in further detail
Many disability consultancies can produce easyread documents for you. A website search will give you options and COI can advise.