What would be the best blade for a reciprocation saw ? I have to cut the old fiberglass shower in half to get it out of the bathroom and replace a new shower. The door is 28 inches wide. The shower is 33 inches wide.

Most Sawzall blades will cut right through, The less the teeth the coarser the cut. Should be pretty quick which ever you use, wood or metal. Try not to breath the fibers.

Strat is correct about using a reciprocating saw or even a jig saw. I use a bi-metal wrecking blade for this task all of the time. Besides the fiberglass getting into your nose, lungs and on your clothes be aware that most shower stalls contain a layer of gypsum. Manufacturers use gypsum to add some thickness and rigidity in the wall panels. When you hit that area there will be a cloud of white dust that makes being in the room unpleasant and unhealthy, wear a good mask and ventilate.

Course ( crosscut ) saw blades are meant for use on wood -- Metal cutting blades

Are fine like most hacksaw blades -- You could probably use either on fiberglass

-- the course blade may jerk more and be harder to control -- the fine blade cuts

Smoother but takes longer -- maybe use a blade between the two...

To eliminate cutting dust -- use a Wet - Dry Vac and suck it up before it gets airborne...

Use a fan to draw dust out of bathroom -- if bathroom has window to outside - open it

And put fan in it blowing outside -- open a window in another room to draw in fresh air

Could use a small Router or Rotozip type tool, set the depth to whatever the thickness of glass is. Won't give any kickbacks if still in place. They don't do inside corners,then you could break or fold in to separate.

Well, with a coarse tooth blade, you will have larger particles to clean up. With a finer tooth blade you will have more dust to deal with. I personally would prefer to use a coarser tooth.
Be sure to use safety glasses, dust mask and heavy work gloves.

I do these so often that I also wear a cheap rain coat taped at the wrists just to keep the fiberglass off me. I also wear a painters hat, I look totally goofy but itch less.