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I recently acquired a 26+ surly instigator (so not quite fat but not entirely skinny) and have been riding it round the local (natural trails). I don't usually bother with mountain bike mudguards but have been getting gobfulls of poo and other farmland cack.
I have seen a few articles like this http://twowheelsbetter.cc/best-of/mountain-bike-mudguards-eight-of-the-best-2/ and they are great but don't really cover 26+ bikes so I have been looking for recommendations. Mainly for the front wheel as there isn't a lot of clearance between tyre and fork.
Are you running rigids up front? If so, there are less options but these will work:
http://www.muckynutz.com/fat_face_fender_black
http://www.muckynutz.com/fat_face_fender_xl_black
Top one is compatible with extremely tight setups. Even works fine on my 405mm a-c street forks with a 26x2.4" Holy Roller. BMX shaped forks also tend to have a lot less room between the legs and tyre so I'd be damn surprised if it doesn't work with Plus size rigids! The XL version needs a bit more space due to its design, but if you're using forks made for large tyres then it ought to be fine.
If you're using a sus fork then you're spoilt for choice. Personally, I'd go with Mudhugger, or if that's a bit too bulky for you then Mucky Nutz regular fenders are great.
Post a pic of your tyre/forks front-facing if you're still unsure.