Classic travertine, all have different sizes. Lovely look. I am currently mad about herringbone, another classic.

Ideally, the larger the slab the more modern it is meant to look (but tell that to medieval floors). I tend to end up almost exclusively with 24/12 size - usually in a 2/3 pattern.

However if it is an imitation travertine, I would definitely advise you lay out your tiles BEFORE tiling to see if you can identify a pattern repeat that would show up. Once it is tiled and you see that repeat, it will be all that you see.