The all-in-one (AOI) Marathon 2 installer has these textures at 512x512 resolution. Install this to enjoy Marathon 2's textures at double that.
This folder is identical content wise to the 'HD Textures' folder contained by default within Marathon 2' plugins folder, except all textures have been replaced by ones with a resolution of 1024x1024.
This differs from the other 1024x1024 texture plugin for M2 on this site in that this download contains Freeverse's HD skyboxes.
To install: unzip, and drop into Marathon 2's plugins folder, then activate it in-game.
IMPORTANT: Delete or deactivate the original "HD Textures" folder.
PLEASE NOTE: To avoid unsightly skybox tiling, play on an FOV of 88 or lower(assuming aspect ratio of 16:9) and tick 'limit vertical view'.