Ocean Accounts, aligned with the UN System of Environmental Economic Accounting – Environmental Accounting (SEEA EA), bring together economic, social and environmental information in a coherent and standardised manner. Ecosystem extent is a structure to understand environmental assets and uses a basic spatial unit to facilitate the classification and measurement of ecosystems by type. This study tested the impact of grid size and method of designation per grid cell for Marine Basic Spatial Units (MBSU), using Saleh Bay, Indonesia as a case study. The extent of mangrove, seagrass and coral reefs were previously delineated in 2021 for ocean accounting activities. This study tested grids with two different cell sizes (10 x 10 m2 and 25 x 25 m2) and two different methods of designation, namely: (i) dominance (extent-based) and (ii) hierarchy (criteria-based) methods. The results indicated that a larger grid size is related to higher error in estimating both total area per ecosystem and spatial configuration within the study area. The dominance method produced more accurate results than the hierarchy method, although, when considering computational trade-offs, a larger grid size and the hierarchy method observed a much lower computational cost. These results demonstrate the need to carefully consider grid size and method when designating basic spatial units for accounting activities, as they impact linked accounting tables and, in turn, have implications when providing information for management and policy.