It needs to match the circuit grounding conductors in size, basically
The clearest indication of the minimum size a box grounding jumper needs to be comes from NEC 250.146 (emphasis mine):
250.146 Connecting Receptacle Grounding Terminal to Box.
An equipment bonding jumper shall be used to connect the
grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 250.146(A) through (D). The
equipment bonding jumper shall be sized in accordance with
Table 250.122 based on the rating of the overcurrent device
protecting the circuit conductors.
This basically requires the box grounding jumper to be at least as large as the minimum size ground conductor permitted for a circuit of that amp rating. Further support for the OP's position, where the grounding jumper to the box must be the size of the largest ground wire coming into the box even if that ground wire is oversized as per NEC 250.122(B), comes from the precise text of NEC 250.148(C):
(C) Metal Boxes. A connection shall be made between the
one or more equipment grounding conductors and a metal
box by means of a grounding screw that shall be used for no
other purpose, equipment listed for grounding, or a listed
In particular, since it basically implies that the jumper between the grounding screw and the equipment grounding conductors coming into the box is an equipment grounding conductor, it means that this jumper must comply with all the equipment grounding conductor sizing rules in NEC 250.122 in order to conform with this section of the NEC.