The National Association for Visually Handicapped (NAVH) provides the NAVH Seal of Approval to commercial publishers for books that meet their large print standards.(Lighthouse International acquired NAVH in 2010). The standards call for:

Maximum limits on size, thickness, and weight
Minimum limits on margins
Type size at least 16 point, preferably 18 point - This is the MOST IMPORTANT POINT
Sans serif or modified serif font recommended
Adequate letter and word spacing
Flexible binding recommended to allow open book to lie flat

Also For many people with visual impairment enlarging the type is not enough. Fonts designed for readability make it easier to distinguish one character from another. Some characteristics of such fonts are:[6]
Sans serif, without short strokes on the ends of characters
Even spacing between letters, so that groups like "ill" don't blend together
Clear descenders on letters such as "j" and "q"
Circular openings on letters such as "b" and "o"
Larger punctuation marks