You need to specify the texture pool size and those associated settings within your .ini files. For developing games, the [your_game]\Config\DefaultEngine.ini file also contains a mirror set of the base properties in the Engine\Config\ folder and should be the copy that is normally modified for your game’s specific settings.
Note that there are independent sets of TextureGroup entries for the Unreal Editor and in-game. These two sets are respectively located in the [SystemSettingsEditor] and [SystemSettings] sections in the config files. This is where you will specify the Texture Pool Size and other config variables.