How to fix a 1/4 inch gap between the tub spout and the wall

If you have access to the same tiles that you used, you could cut them to size (you may want to consider using a good cutter or wet saw). That way you can caulk the space in between the tile and the acrylic shower base. If you don't have any extra tiles or can't find any more of the same type, you could find a nice complementary looking small tile and install that instead.

Typically you would use a copper sweat by FIP 90° elbow (often called a wing back elbow, as it has tabs on it to allow for securing to backing). Then screw in a test nipple (or use a nipple and cap) while the tile is being done. Once tile is done, use a brass nipple of the correct length (best to buy a few of different lengths and just return the ones that were the wrong length).