We studied the aggregation of magnetic particles into chain-like structures in a cobalt-based magnetic fluid, exposed to external magnetic fields. The length of chain segments in very strong magnetic fields of up to 2 Tesla was measured in situ using small angle neutron scattering. Arrangement of the magnetic particles was studied with the scattering vector Q^→ over right arrow aligned parallel to the magnetic field lines, and at angles of 30° and 60° with respect to B^→. Although chains several hundred particles in length were predicted, we observe a maximum correlation length of ~65 nm equivalent to 3-4 particles up to highest fields. We speculate that the interaction between chains, i.e., the interplay between the entropy and energy of the system combined with the particular properties of the magnetic dipole-dipole interaction ultimately decide the length of the particle chains.