We examine the radial scales developed inside field line resonances (FLRs) when they are driven by both broad and narrow frequency bandwidth fast mode sources. The finest FLR radial scales are always limited by ionospheric dissipation, being determined primarily by the height-integrated Pedersen conductivity &Sgr;P. We estimate likely FLR radial scale sizes in both dayside and nightside ionospheric conditions, and confirm the accuracy of these estimates using the wave Doppler shifts observed on inbound/outbound passes of Active Magnetospheric Particle Tracer Explorers CCE [Anderson et al., 1989>. Dayside broadband FLR events can have radial scale lengths significantly shorter than their overall widths, suggesting they may possess several radial amplitude nodes and antinodes. Further, we examine the Kelvin-Helmholtz (KH) stability of FLRs due to their azimuthal velocity shear. We estimate a FLR toroidal velocity threshold, for particular &Sgr;P, beyond which the KH growth rate is sufficiently large to disrupt the FLR. For typical magnetospheric conditions, FLRs are not likely to be disrupted by driving secondary KH vortices. For large-amplitude FLRs in regions of high &Sgr;P, however, it may be possible for FLRs to be disrupted by the KH instability and to develop into large-scale KH vortices. We further speculate on the possible link between auroral zone FLR internal radial scales and the observed optical widths of discrete auroral arcs. ¿ 1997 American Geophysical Union