What can I do to maximize the space in my bathroom

Both my bathrooms are small and have no room for linen storage either. I have in baskets in my closet on a shelf.

can you use a bathroom shelf behind the toilet

There is a shelf set that is spring loaded that goes over your toliet back. Metal or plastic. Works well...

Use a back-of-the door hanging towel rack. Then think up. Put a shelf over the door from wall to wall for extra towels, etc. If you have a knack for construction, you can also open up a wall and construct shelves between the studs. This will give you between a 4-6 inch depth for shampoos, etc. Good luck.

One of my bathrooms is pretty small too and it goes with the master BR. It also only has a shower instead of a tub. I hang towels over my shower door to dry before adding them to my wash basket. I also made them a priority to put the clean ones in my bottom cupboards. I took everything out, cleaned it well and put down vinyl liners that you can find at Walmart, Target or any of the other big box stores or perhaps the dollar store. I had to find a home for other items that were under there so we found a larger "medicine" cabinet for some of those items. The other items just had to be removed from the room. One of my thoughts that I'm working on is to hang a clear shoe organizer over the door on the inside. I haven't decided if I'm going to hook it over the top like the way it's made or if I'm going to cut it off at the top and nail it directly to the door, but it will give me more storage space. I also like the shelves above the toilet idea. Just don't make the first shelf so close that you can't take the lid off of the tank. ;) Wishing you the best.