Depending on what you are going to clad the frame with you can get away with very thin timber for the frame, 25mm or even a bit less, so you could use 2x1 which is readily available. You will be normally be very limited by how much space you have under the rim of the bath which will limit the size of timber you can use. There is no need to use treated timber but if you are planning to use MDF for the panels make sure you use moisture resistant boards.