Coiled-tubing drilling (CTD) provides a compact surface footprint and superior downhole pressure control compared with conventional jointed-pipe drilling. However, the flexible tubing and the wellbore tortuosity generated by the downhole bent motor severely limit its lateral reach. Traditional mitigation methods, such as friction reducers and agitators, deliver only marginal improvements. Continuous rotation of the downhole bent motor eliminates drilling direction bias and offers an alternative solution for extending CTD lateral reach. This approach creates a helical wellbore with a central straight conduit that allows the BHA and coiled tubing to pass along it without bending, significantly reducing tortuosity-induced friction and drag. The diameter of the central straight conduit is a key parameter controlling stuck-pipe risk. This work develops a predictive model, validated by advanced drilling dynamics simulations, to quantify helical path characteristics and central conduit size as a function of motor bend angle, drilling rate of penetration (ROP), and the speed of the continuous motor bend rotation (RPM). First, a remarkably simple analytical model is derived to predict the helical wellbore geometry. The helical pitch is governed solely by the ROP/RPM ratio, while local curvature is driven primarily by continuous rotation, which forces the drilling direction to change steadily with depth. Combining these relations yields a closed-form expression for the diameter of the central straight conduit. To validate this closed-form expression, we perform advanced transient drilling dynamics simulations. The simulation tool was previously validated for numerous challenging scenarios. We obtain excellent agreement between the simulation results and the closed-form expression across a wide range of ROP, RPM, and motor bend angles. The main contribution of this work is a fast, physics-based formula that predicts the size of the central straight conduit as well as the helix path geometry from operating parameters, enabling pre-job and real-time RPM/ROP selection to guarantee sufficient clearance for safe CTD operation with extended reach. This establishes continuous rotation as a practical and controllable solution for improving CTD lateral reach.