Quantitative size measurements of single linear and circular DNA molecules in nanofluidic slitlike confinement are reported. A novel experimental method using DNA entropophoresis down a nanofluidic staircase implemented comprehensive variation of slitlike confinement around d approximate to 2p, where d is the-slit depth and p is the persistence length, throughout the transition from strong to moderate confinement. A new numerical analysis approximated and corrected systematic imaging errors. Together, thee advances :enabled the first measurement of an experimental scaling relation between the in plane radius of gyration, R-||, and d, yielding R-|| similar to d(-1/6) for all DNA samples investigated This differs from the theoretical scaling relation, R-e similar to d(-1/4), for the root-mean-square end-to-end size, R-e. The use of different labeling ratios also allowed a new test of the influence of fluorescent labels on DNA persistence length. These results improve understanding of the basic physical behavior of polymers confined to nanofluidic slits and inform the design of nanofluidic technology for practical applications.