It took some digging to find this article from last year.
http://www.racer.com/imsa/item/1187...7-prototype-regulations?showall=1&limitstart=
A "length from 535mm (4-cylinder) to 685mm (V8), width from 370mm (4-cylinder) to 890mm (V10), height from 500mm (4-cylinder) to 752mm (V8)," and "crankshaft height from 77.5mm (V8) to 120mm (V8)" is listed, and within those dimensions, IMSA will have plenty to offer a wide variety of auto manufacturers.
Weights for 4- and 6-cylinder engines have yet to be defined, but V8s (150-180kg) and V10s (260kg) are noted. V12s, like the ones found in Aston Martin's GT3-spec Vantage, rotary engines, and diesels are not listed as options. Turbocharging is only mentioned as options for 4- and 6-cylinder engines.
Maximum engine displacements will also govern which manufacturers can participate in 2017. A 2.5L limit for 4-cylinder turbos, 4.5L limit for 6-cylinder turbos, 6.2L limit for NA V8s, and 5.2L limit for NA V10s is currently in place. Based on last two limits, Chevy LS3 V8 crate motor and Audi's R8 V10 could have prompted the respective 6.2L and 5.2L limits.
The Blancpain GT series specs have the SLS listed as 6208cc and AMG calls it a 6.3L so I guess the V8 size limit has been increased.
This is awesome news and hopefully it will be on track before mid season next year.