The growing penetration of renewable energy resources such as wind and solar into the electric power grid has changed grid dynamics. Particularly, the large-scale displacement of synchronous generators with inverter-based resources significantly reduces the short-circuit strength and thus increases the risk of the small-disturbance stability issues, such as sub/super-synchronous oscillation issues, resulting from fast-acting inverter controls and the power network. It is challenging to assess grid strength in terms of small-disturbance stability in power systems in high penetration level of inverter-based resources due to the complex interaction between power networks and different inverter control configurations such as grid-forming and grid-following inverters. It becomes more challenging for the 100% inverter-based power system, where all synchronous generators are displaced with inverter-based apparatus. In such a power system, the commonly used methods for grid strength assessment are invalid due to their assumption that synchronous generators are used to provide voltage support for grid strength assessment. To address the challenges, this paper presents a method for assessing grid strength in terms of small-disturbance stability in a 100% inverter-based power system. First, we reveal voltage support characteristics of grid-forming inverters. Then, we further analyze the small-disturbance stability of a multi-inverter system of grid-forming and grid-following inverters. On this foundation, a method is proposed by extending our previously developed generalized short-circuit ratio to assess grid strength in terms of the small-disturbance stability of the 100% inverter-based power system while considering the complex interaction between grid-forming and grid-following inverters coupled by the power network. The efficacy of the proposed method is demonstrated on a modified two-area-four-machine system and a modified IEEE 39-bus system via the eigenvalue analysis and electromagnetic transient simulation.