To scale a tileset you must:
1) Copy the original tileset directory to a new directory.
2) Scale the graphics.
3) Modify all the .spec files in your new tileset directory to math your graphics files (hint: variables x_top_left, y_top_left, dx and dy).
4) Create a copy of the tileset definition (create a copy of the corresponding .tileset file).
5) In the .tileset file change the values of the following variables to match the size of your tile set; normal_tile_width, normal_tile_height, small_tile_width and small_tile_height.
6) In the .tileset file change the values of the following variables to adjust the city and unit position; unit_flag_offset_x, unit_flag_offset_y, city_flag_offset_x, city_flag_offset_y, unit_offset_x, unit_offset_y and citybar_offset_y.
7) In the .tileset file you can also change the fonts if necessary.
I think that is all.
I recommend you that if you change the width/heigth relation in a hex tileset disable the terrain blend in the .tileset file (for adjacent terrains) because sometimes the terrain blend is not between the correct tiles (some months ago I was experimenting with a new hex tileset for Freeciv 2.1 with double width and same heigth of the original).