Smaller the better. Make sure you have a good tile guy to install 12×24. You will need to pay more but it will be worth it.

Is it more expensive to have smaller grout ??

I am tile installer and the grout size has nothing to do with the price, different tile size has different prices

Definitely smaller... especially if it's on the floor!

You can get bad lippage when installing 12×24 if not properly installed. Make sure you have a very experienced tile setter.

Listen to Creative Tile on this question! Also make sure you get a quality tile, since 12" x 24" is a very large tile and I've seen a lot of breakage with cheaper tile done by inexperienced tilesetters.

What size grout line for 24x24 tile?

Literally not 1 answer for the question.

The answer to the question is in my previous comment from 3-1/2 years ago. You can only go as small as the actual tile purchased allows. 12x24 means nothing. Literally:)

So there is no standard for grout size based on tile size? Grout size is determined entirely by manufacturer's recommendation?

I think many of us expect some kind of guide or standard about this kind of thing but I guess a definitive answer can't be given because there is so much variation in tile quality. For example: A higher quality tile could allow for a 1/16" or 1/8" grout line and maybe even a brick layout as lippage may not be an issue (in a perfect tile world) where as a lower quality tile might have major lippage risk so laying the tile in 1/3's and having a 1/4" grout line is recommended.

Got it!

Ideal depends on type of tile, and uniformity of same. For example, if you are using hand made ceramics, there will be high variation, so you can’t use small lines as they exaggerate differences in thickness And overall tile dimension.

The manufacturer should know how much variation is in their product, though, so if they recommend a 7mm line for example, you can go larger as your aesthetic likes, but if you go smaller you risk a sloppy looking install.

I did a tile floor with a grout line of 3/16 but got grout for joints from 1/16 to 1/8 what do I need to use for the 3/16

if your tile is Rectified 1/16 joint if not 3/16 because then your tile can range in size

1/16 joint if tile is rectified if not 3/16 because then your tile can run different sizes