Need a bit more info really! Couple of questions:
Unbraked I take it?
Uk trailer or from abroad?
Trailer manufacturer may be good?
If not post a pic....
If it's the most common unbraked trailer hubs (cast, 4 x 3/8" studs on a 4" PCD) then your bearings are likely to be taper roller bearings, same size on both inner and outer (outside bearing size 50mm, inside diameter 25mm). Bearing numbers likely to be:
Inner cone: L44643L (seal attached)
Inner cup: L44610
Outer cone: L44643
Inner cup: L44610
However, it's often as economically viable to replace the entire hub and bearing assembly together. You should pay around £20 ish for an entire hub, including bearings.
If the above is a load of balls then post up a pic.
