The thing nobody tells you about bathroom tile is that you don't actually notice it day to day until the grout starts to go. You notice the grout. A beautiful tile with dirty grout lines looks worse than a plain tile with clean ones, every single time, and nobody mentions that part when you're standing in the showroom holding a sample.
If you've got nothing to go on yet, start with a large format porcelain tile on the floor and a simple rectangular tile on the walls, something in the twelve by twenty four range for the floor and a basic subway or a slightly longer plank shape for the walls. Fewer grout lines means less cleaning and less to go wrong, and porcelain handles water sitting on it for years without drinking it in the way some stone will. For color, the easiest finishes to live with long term are soft warm whites, light grays with a bit of beige in them, and anything described as greige. They don't scream a decade at you the way a true cool gray or anything heavily veined and dramatic will. Matte finishes hide water spots and soap scum far better than gloss, which looks incredible in photos and shows every single drip within about a week. Glossy white subway in a small windowless bathroom with yellow toned lighting looks clinical fast, almost sterile, like you're about to get a filling rather than take a shower.
High contrast patterned tile, the black and white hex or anything with a strong geometric print, is gorgeous in isolation but clashes badly with busy vanity hardware, patterned shower curtains, or anything with its own strong personality. Pick one loud element in the room and let everything else go quiet around it. A heavily veined marble look floor fights with colored cabinetry almost every time I've seen it tried.
For sizing, measure the actual square footage of the floor and walls you're tiling and add ten to fifteen percent for cuts and breakage, more like twenty if the room has a lot of odd angles or a tricky toilet flange to work around. Bring the sample tile home and hold it up under your actual bathroom lighting before you commit, because a tile that looks warm and inviting under store lighting can turn a flat, sad gray under a cheap bathroom fixture. The biggest mistake is sizing tile to the room on paper without accounting for the shower niche, the vanity backsplash, and the transition strip at the doorway, all of which eat into your total and need planning before the order goes in, not after the tile guy is standing there with half a box left.