The hole is only about 30cm across. It looks smaller because it's hard to appreciate that the tomb is 3m wide and 20 odd m long. I think a skinny kid or even a skinny shamen could climb in and out with a bit of practice.

Have a look around at some of the French Allee Couverte that are very similar and you'll see their port hole entrances can be smallish too.

It's an amazing place and anyone going to the Frankfurt area with time to spare should definitely head out to it.