Best size? Whatever is comfortable for you. Most people use 1 unit per 1 meter as rule.
Not sure what you are asking in your second question. Larger numbers equals greater size in game. Scale doesn't usually affect speed much. Mostly higher vertices, polygons, surfaces affect speed.
Scale of texture should be related to how 'close' you will be seeing the texture in game. If it will always be far off in the distance than there is less of a need for high resolution textures. Smaller textures of course, increase speed.
Not sure what you are asking in your second question. Larger numbers equals greater size in game. Scale doesn't usually affect speed much. Mostly higher vertices, polygons, surfaces affect speed.
Scale of texture should be related to how 'close' you will be seeing the texture in game. If it will always be far off in the distance than there is less of a need for high resolution textures. Smaller textures of course, increase speed.