Magnetic nanoparticle (mNP) based thermal therapies have demonstrated relevance in the clinic, but effective application requires an understanding of both its strengths and limitations. This study explores two critical limitations for clinical use: (1) maximizing localized mNP heating, while avoiding bulk heating due to inductive coupling of the applied field with the body and (2) the limits of treatable volumes, related to basic heat transfer. Two commercially available mNPs are investigated, one superparamagnetic and one ferromagnetic, thereby allowing a comparison between the two fundamental types of mNPs (both of which are being evaluated for clinical use). Important results indicate that in dispersed solutions, the superparamagnetic mNPs outperform on a per mass basis (2× better), but the ferromagnetic mNPs outperform on a per nanoparticle basis (170× better), at the fields of highest clinical relevance (approximately 100 kHz and 20 kA/m). We also demonstrate a new method of observing heating in microliter droplets of mNP solution, leading to scaling analyses that suggest treatable tumor volumes should be ≥2 mm in diameter (for mNP loading of ≥10 mg Fe/g tumor), to achieve therapeutic temperatures ≥43 °C. This technique also provides a novel platform for quantifying heating from microgram quantities of mNPs.