A radio access network (RAN) determines whether transmission time interval (TTI) bundling is warranted for a wireless communication device (WCD) based on at least a power headroom report from the WCD. In response to determining that TTI bundling is warranted, the RAN selects a TTI bundle size, N, for the WCD and instructs the WCD to use the selected TTI bundle size. The TTI bundle size, N, could be selected from among a set of predefined values, such as N=2, N=3, and N=4. The selection could be based on various factors, such as a utilization of an uplink shared channel, a re-transmission rate of the WCD, and/or a remaining battery life of the WCD.