Use 18 x 18. I used that size in my small bath and it looks great. I think 12 x 12 looks dated.

12x12 is easier to work with if you are doing it yourself. Some tile cutters may not cut 18" but a tile pro would have the larger saw needed or you can rent saw from HD. As far as style, it may or may not be dated to see 12x12 tile on the floor. I think it all depends what look you are going for. If you are trying something very contemporary than the 18x18 would be fine. I redid one of my farmhouse bathrooms and cut down slate to 6x12 and laid a herringbone pattern (its not a true 6x12, as there is adjustment to the size to accommodate the grout width) The room is a 6-1/2x9 full bathroom. Very country feel to me:

I used 18 x 18" in my small bathroom and I love the way it looks.

I would have to see a floorplan to decide. The larger the tile the more important the tile layout.

I've seen small bathrooms look great with larger format tile and I've seem them look awful, if the tile layout is not pleasing. And in some bathrooms, because of fixture and door location, it's very hard to get a large-format layout that works in all directions.

To me it would depend if you ended up with slivers of large tile somewhere because of the floorplan. That would be a negative for me: "current" vs. "dated" becomes irrelevant compared to how it Works in a Particular bathroom.

Both my DD and I both have small bathrooms with large format tiles so I thought it might be helpful for you to see a couple photos.
My DD bathroom floor area is only 76x61 and she used 24x24 porcelain marble look tiles. I think the larger tiles actually make the room look bigger and there are less grout lines to deal with.

I am not sure the actual size of my own bathroom but the floor area is probably less than your 65 sq. ft. I used 18x18 set on the diagonal.

When we were deciding bath floor tile, I drew the options out on paper to see what it would look like. Then I gave up on the paper and used a drawing program on my computer. In the end we did 13 x 13 tiles set diagonally in our long, skinny hall bath and 12 x 12 tiles set the usual way in our small master bath. Been happy with both decisions.