  We report a systematic study on the structural and magnetic properties of
Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between $5$ to $25$ nm,
prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large
magneto-crystalline anisotropy of the synthesized particles resulted in high
blocking temperatures ($42$ K \leqq $T_B$ $\leqq 345$ K for $5 \leqq$ d $\leqq
13$ nm ) and large coercive fields ($H_C \approxeq 1600$ kA/m for $T = 5$ K).
The smallest particles ($<d>=5$ nm) revealed the existence of a magnetically
hard, spin-disordered surface. The thermal dependence of static and dynamic
magnetic properties of the whole series of samples could be explained within
the N\'{e}el-Arrhenius relaxation framework without the need of ad-hoc
corrections, by including the thermal dependence of the magnetocrystalline
anisotropy constant $K_1(T)$ through the empirical Br\"{u}khatov-Kirensky
relation. This approach provided $K_1(0)$ values very similar to the bulk
material from either static or dynamic magnetic measurements, as well as
realistic values for the response times ($\tau_0 \simeq 10^{-10}$ s).
Deviations from the bulk anisotropy values found for the smallest particles
could be qualitatively explained based on Zener\'{}s relation between $K_1(T)$
and M(T).
