Is it better to use large-format tiles or small tiles?

Large tiles are more practical if the wall is pretty regular, since they need fewer grout joints and dirt doesn't build up as much in the gaps. But if your wall is a bit crooked or uneven, small tiles are easier to lay because they compensate better for imperfections. The real problem with large tiles is that if you mess up the first placement you'll have a hard time correcting it, whereas with small tiles you've got more room for error. In the end it depends on your surface conditions: smooth and straight surface = large tiles, imperfect wall = small tiles.

What spaces do you have and how much time can you dedicate to laying them? Large tiles have an advantage that doesn't get mentioned often enough: with fewer grout lines you risk fewer water infiltrations over time, so if you're tiling a bathroom or kitchen it's really important. But it's true they require the wall to be almost perfectly prepared, otherwise you'll see every tiny unevenness. Small tiles are more tedious to lay (more work, more mortar, more waiting) but they forgive mistakes way better, so if your wall is slightly out of square or just irregular, you'd better go with them. It depends on how actually crooked your wall is: if we're talking a few millimeters, large format works fine with a good screed, but if the irregularities are more pronounced, small is better.

I put large format tiles in my place, and honestly it really depends on what you're tiling and what condition the substrate is in. Whoever says large tiles only work on a perfect wall is right, but there's another thing to consider: installation is a nightmare, and if it's not done well (especially with large tiles) you'll see the results right away. With small tiles, though, you've got way more margin for error, so if you're not a stickler for precision or you don't call someone who really knows their stuff, it's definitely the safer choice.

About the water seepage issue someone mentioned: yeah, fewer grout lines = fewer water problems. But that really only matters if you're doing a bathroom or kitchen where there's high humidity. For a hallway or bedroom it's pretty much irrelevant. And then the joints between large tiles, if you don't seal them properly, they'll cause problems anyway. Small tiles have the advantage that even if the sealing isn't perfect, any potential damage is more localized.

I'd also add something practical: large tiles are heavier and a pain to work with. And if you need to cut any, you need better tools. It's not impossible, but if you're not in a rush and want to avoid headaches, small tiles save you time and stress.

How much are you willing to spend on labor? That's where it really comes down to! Large tiles cost less to install if the wall is decent, but if you need to do leveling and corrections maybe with special mortars, the cost shoots up fast. One thing nobody's mentioned: try asking the tiler to use adjustable spacers even with large tiles, not just fixed ones - it lets you play better with the wall's irregularities without having to do crazy prep work. If your budget's tight, often small tiles actually work out better for you for this exact reason, fewer surprises during installation!