Mine is a big bearing, but apparently the axle shaft diameter is the same for both. I did a similar post several years ago and several members responded that the shafts are the same it is just the OD diameter of the bearings that is different.
Axle shaft diameters are different between big and small-bearing applications.
Timken 514003 (big-bearing)
Ø3.150" OD x Ø1.531" ID x 1.083"
Timken RW207CCRA (small-bearing)
Ø2.835" OD x Ø1.378" ID x .845"
Edited to add SET20 aftermarket tapered roller bearing option:
Timken SET20 [A20/BR20] (big-bearing)
Ø3.150" OD x Ø1.563" ID
Please note that there are also aftermarket sealed ball bearings with the same ID as the SET20 bearing noted above, Ø1.563", though most of these bearings will look a little different due to having O-rings in the outer race of the bearing and seals that are rated for both oil and grease, implying that they can be installed and run without inner oil seals. That said, I would still run an inner oil seal with any sealed ball-bearing setup as a belt and suspenders type of option. Some people will do a similar solution with the SET20's, noting that while they're intended to be run without inner seals (just the external ones captured on the axle by the bearing) and lubed by the diff lube, some people will run an inner oil seal and then grease the bearing like you would a front wheel bearing to avoid oil leaking issues.
