Submit in only one category at a time: fiction, non-fiction, and art will be considered for both their print and online editions, unless you specifically state otherwise in your cover letter. Actually, they remain open to submissions of photography and art year-round.
Granta only publishes original material, i.e. first-ever publication. There is no set maximum length or minimum length, though most of their submissions fall between 3,000-6,000 words.
Include a cover letter stating where your work has been published before, if relevant.
Do not submit book manuscripts, academic essays or reviews. Only submit work written in English.
Get more details and submit through their website http://granta.com/
You can read some of the fiction on their website, to get a feel for the style and content. Send only your best work.