Uh yeah the weight is a HUGE issue!!

12 x 24 s barely make it with good mortar. That is 2 sq feet

20x20 are closer to 3 sq ft

Odds of 10 lbs staying up on the wall with mortar is not good.

All I can do to lift 18 x 18 floor tile around - forget putting them on the wall

Guessing those are floor tile.

Floor tile are thicker than wall tile typically more like 3/8 -1/2 inch vs wall tile at 3/16 -1/4 inch- they weight more and are harder to get to stay.

I've done - used floor on wall -but never in more than 6x6

If they are floor tile and that large doubt ANY contractor will mess with it. Tiles won't stay and you'll be screaming at them

WHy not cut the tiles to more manageable dimensions.

That is what wet saws were made for - but to get one big enough for those things is gonna be $200-300. A manual cutter can handle 20" tiles and at least do the first couple cuts

Assuming they are the same thickness, the total weight of 20 x 20 tiles won't be any more than the total weight of 12 x 24 tiles. The walls can handle it. Whether or not the installer can, is a different matter. The questions you ask are the ones that your installer needs to answer for you. We can't see your layout from here, so we can't advise on how much waste there will be. If you have a typical 60" tub, then there very well might be less waste with the 20 x 20, assuming that you were originally intending to install the 12 x 24 tiles horizontally.

I agree with Ann - those are heavy tiles - floor tiles are usually thicker than wall tile. I'd use 12 or 13" tile.

Thanks for the comments. The 20 x 20 (according to the specs) is 9 mm thick, 10 lbs and the 12 x 24 is 10 mm thick, 8.75 lbs.

Both are way too heavy to try to put up on a wall.

It is not whether the "walls can handle it" - it is whether there is any mortar out there that will hold that much weight to the wall and not eventually let go so you have falling tiles.

Those both are the thickness of floor tile

You have to stop trying to use floor tile on the wall.

Wall tile = 1/4 in (6mm) or less.

Typical porcelain tile in the 5/16" to 3/8" thick range will weigh in at roughly 3-1/2 to 4 lbs per sqft.

Your 20x20s weighing 10 lbs? They fall right into that range. So your first question, no, the tiles are not too heavy for the walls. Or for the modified mortar used to adhere them to the walls.

As to installation issues, yes, the flatter and more plumb the walls are, the easier the installation. Thinset coverage? The installer will want to set a tile and then pull it off the wall, check thinset coverage, and adjust the trowel size if needed. If the walls are flat he can use thinset. If the walls are wonky and a thicker setting bed is required to keep the face of the tile in plane, then the installer can move on to a medium-bed type of mortar.

Setting mortars come in low-slump formulations to help prevent tile creep, but shims and tape can be used as well if needed.

Tile waste? Figure out a pattern for your exact wall size considering the exact tile size and the exact grout lines you want. In a perfect world with 20" tile, three tiles on the back wall and 1-1/2 tiles on each side wall will cover a 60x30 tub surround nicely.

Added labor with large format? It's all in the prep. If the walls and corners are plumb and in plane, then the installation may be a breeze. If the walls are wonky, then to get a great installation, the pace of installation can slow dramatically.

I used 18 x 18 porcelain tiles in a shower and on the ceiling as well. Its been in for 5 + years and no issues. They were fairly thick and heavy too. My tile guy wanted the smaller ones but they were back-ordered so we went with the larger size. Shower was 6'5" by 36"

I used a 24" x 24" marble-look porcelain tile on my shower walls and my tile guy had no problem installing them. He looked more worried when I asked him to install a small mosaic on all the wall in a powder room!

Thinsets have a minimum rating of 50 pounds per sf sheer. Many have a higher rating. We install 24x24's all the time. Larger is getting more common.

I agree. Weight has little to do with your question. Tiles on the wall can be quite heavy as "Sheer Strength" is the issue here. Any properly built wall can handle FAR more weight than you might think. Look...they aren't on the ceiling where "drop strength," for lack of a better term, can come into play. On a vertical surface, if you could find some way to grab onto it, it would support your weight....and more. Let not your heart be troubled....and tile on!