Grass snakes Natrix natrix feed mainly on amphibians, especially frogs and toads. They do stray from this diet occasionally, eating fish, small mammals and, more rarely, birds, but nothing as large as a chickens.
The bigger the serpent, the larger the prey it can take, and most adult grass snakes are 75-90cm. They do not kill their victims using constriction or venom, but ingest it alive.
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Therefore, they are not only limited by the size of the prey they can swallow - they also have to be able to overpower it.
A small adult grass snake is capable of swallowing a fully grown common frog, and a big specimen would certainly be able to tackle the largest female toad.