Equipment Bonding Jumpers are sized per section 250.102 in this case 250.102 (C) Size — Equipment Bonding Jumper on Supply Side of Service. I would size the jumper per table 250.66 (#4CU or 2 AL).

The metal riser at the pole should be bonded in accordance with section 250.92 (A) using a method described in 250.92 (B). Part (A) requires that the metal raceway enclosing service conductors be bonded. And the method in part (B) States that electrical continuity at service equipment, service raceways, and service conductor enclosures, shall be ensured by one of the following methods, ( 1-4):
(1) Allows bonding to the grounded service conductor, (2) allows connections utilizing threaded couplings etc, made up wrench-tight. The third (3ed) provision recognizes threadless couplings and connectors, and the fourth (4th) requires approved devices, such as bonding-type locknuts and bushings
Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.

250.97 will require one of these four methods be employed where the voltage to ground exceeds 250V. (There is an exception)

I believe if you read the Code 250.102 (C) establishes the size of the bonding conductor on the line side of the service equipment, which would require a No. 4 CU or No. 2 AL to the metal raceway (yes a bonding bushing will work but so would a pipe clamp, etc.). I will stand by to read other opinions.