how big the luggage has to be in order to take with me in greyhound?

i have exactly this one its a 28′ http://bigduffles.com/duffel%5Fbags/monster%5F22/#

would it be allowed?

Have you bothered to check Greyhounds website? Greyhound has the normal 62″ limit (length+width+height) for checked bags. If yours is the one pictured it will be accepted.

Rolling Duffel Luggage

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