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Typically, magnetic iron oxide nanoparticles (MNP) with core diameter in the range of about 16 nm to 22 nm are accessible by magnetorelaxometric (MRX) measurements at room temperature whereas the relaxation of smaller particles is too fast to be observed with a conventional MRX setup. In order to extend the size limitation towards smaller particles, we suggest applying temperature dependent magnetorelaxometry (TMRX). In this study, we outline and validate the procedure experimentally for temperatures between 5 K and 200 K on in-vitro preparations of MNP using a conventional MPMS SQUID magnetometer. On this basis, we applied TMRX for the in-vitro quantification of small sized MNP uptake by tumor cells, i.e. on HeLa and Jurkat tumor cell lines, reaching a detection limit of about 100 ng. We further showed that TMRX signals are characteristic for particular MNP preparations, opening the possibility to observe changes in the particle size distribution during the uptake of MNP by a biological system.