To address the complex static instability mechanisms and the difficulty in evaluating maximum active power transmission for grid-forming (GFM) converters in resistive-inductive (RL) grids, this paper proposes an analytical method based on \"Power-Voltage Vector\" geometric planes. This approach utilizes geometric vectors to characterize active power boundaries under multiple electrical and control constraints, providing an intuitive visualization of static instability mechanisms to guide control parameter design. Specifically, voltage/current and control constraints are first characterized in the voltage plane, and the feasible region is then lifted and projected onto the virtual power plane. Based on the rotational projection relationship between actual active power and virtual power, the actual active power boundary in RL grids can be intuitively characterized. Furthermore, a geometric optimization method for control parameters is developed for various operating scenarios, including different grid strengths and R/X ratios, to achieve maximum active power transmission and parameter configuration. Finally, offline simulations and hardware-in-the-loop (HIL) simulations are carried out under typical scenarios, such as inductive and RL grids, to analyze static instability mechanisms and active power limits, verifying the correctness of the proposed method.