The first thing that needs to be done is to decide on a theme for the entire bathroom. A few popular themes are Asian, out house, rustic and sand & sea. It is important to find a theme that appeals to you and stick with that theme for the entire bathroom. While you are choosing art prints the rest of your space needs to be considered too. Using multiple themes for one bathroom can end up looking thrown together and not orderly. Since bathrooms are usually smaller spaces this is very important.
Once a theme is determined then the next thing to do is choose art prints that you are happy with and that fits your theme. When choosing the art prints it is important to stay within the theme, but also to find art posters that are somewhat similar in color tones. They do not have to all be the same color, but if you are going with bright colors then keep to that color palette, the same if you go with warm colors, etc. When choosing the color theme consider if you are planning on painting your walls in the same tones and which color will fit with the tone of your furniture, counters, sink and shower.
Now that we have a theme and a general color range lets pick the art prints! First identify the areas on your walls where you will want the art posters to hang. Some areas may be large and will either require a very large piece of art or a collection of smaller items. In this case I recommend a collection of smaller items, it gives a more complete feel for most bathrooms. Art prints of course come in all shapes and sizes, but one thing to consider when you find a print that may fit in an area of your bathroom space is the additional width and height framing will add to the item. Framing can be done with a small matte so not to add much size to an art print, but it can also be done with a wide matte as to add quite a bit of size to the item. So, when picking art posters for your bathroom space just remember framing and matting can change the size considerably to fit different areas.
In the bathroom you definitely want to follow the general rule of thumb for hanging art prints. The general rule is to hang art posters at eye level, no matter how high the ceiling is. This will give your bathroom a feeling of organization and format, even if it is a small space.