Intramolecular hydrodynamic interactions (HI) in flexible polymer chains influence both the equilibrium and nonequilibrium physical properties of macromoecules. In this work, we utilize a combination of single molecule experimental techniques and Brownian dynamics (BD) simulation to investigate the role of HI and excluded-volume (EV) interactions for DNA molecules ranging in contour length from 150 to 1300 mum. Epifluorescence microscopy is used to directly observe the dynamics of DNA molecules in planar extensional flow, and a semiimplicit bead-spring BD algorithm with fluctuating HI and EV interactions is presented. Quantitatitative agreement between ensemble average transient molecular extension in experiment and BD simulation is shown for DNA with 150 mum contour length. Simulations show polymer conformation hysteresis for larger DNA chains (1300 mum in length) when HI and EV parameters are chosen such that simulation results match the experimental polymer relaxation time and polymer stretch at flow strengths below the coil-stretch transition. Furthermore, conformation-dependent resistivities are extracted from BD simulation for DNA chains 1300 mum in length, and this drag functionality is utilized in a coarse-grained Brownian dumbbell model with variable resistivity. Finally, steady-state molecular extension results from the coarse-grained model are compared to simple polymer kinetic theory for a dumbbell with variable resistivity.